diff --git a/packages/destination-actions/src/destinations/klaviyo/__tests__/multistatus.test.ts b/packages/destination-actions/src/destinations/klaviyo/__tests__/multistatus.test.ts
index 340f90df9df..8c842b86961 100644
--- a/packages/destination-actions/src/destinations/klaviyo/__tests__/multistatus.test.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/__tests__/multistatus.test.ts
@@ -257,6 +257,48 @@ describe('MultiStatus', () => {
         errorreporter: 'INTEGRATIONS'
       })
     })
+
+    it('should reject events where external_id exceeds 255 characters', async () => {
+      nock(API_URL).post('/event-bulk-create-jobs/').reply(202, {})
+
+      const longExternalId = 'a'.repeat(256)
+      const events: SegmentEvent[] = [
+        // Event with external_id exceeding 255 characters
+        createTestEvent({
+          type: 'track',
+          timestamp,
+          properties: {
+            external_id: longExternalId
+          }
+        }),
+        // Valid Event
+        createTestEvent({
+          type: 'track',
+          timestamp,
+          properties: {
+            email: 'valid@gmail.com'
+          }
+        })
+      ]
+
+      const response = await testDestination.executeBatch('trackEvent', {
+        events,
+        settings,
+        mapping
+      })
+
+      expect(response[0]).toMatchObject({
+        status: 400,
+        errortype: 'PAYLOAD_VALIDATION_FAILED',
+        errormessage: 'Length of external_id must be no more than 255 characters.',
+        errorreporter: 'INTEGRATIONS'
+      })
+
+      expect(response[1]).toMatchObject({
+        status: 200,
+        body: '{}'
+      })
+    })
   })
   describe('addProfileToList', () => {
     beforeEach(() => {
diff --git a/packages/destination-actions/src/destinations/klaviyo/addProfileToList/generated-types.ts b/packages/destination-actions/src/destinations/klaviyo/addProfileToList/generated-types.ts
index 669a43d79ab..cc9a70db301 100644
--- a/packages/destination-actions/src/destinations/klaviyo/addProfileToList/generated-types.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/addProfileToList/generated-types.ts
@@ -14,7 +14,7 @@ export interface Payload {
    */
   list_id: string
   /**
-   * A unique identifier used by customers to associate Klaviyo profiles with profiles in an external system. One of External ID and Email required.
+   * A unique identifier used by customers to associate Klaviyo profiles with profiles in an external system. One of External ID, Email or Phone Number is required. Must not exceed 255 characters.
    */
   external_id?: string
   /**
diff --git a/packages/destination-actions/src/destinations/klaviyo/config.ts b/packages/destination-actions/src/destinations/klaviyo/config.ts
index a41f7ffb36c..925f84a8e05 100644
--- a/packages/destination-actions/src/destinations/klaviyo/config.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/config.ts
@@ -2,6 +2,7 @@ import { KLAVIYO_REVISION_DATE } from './versioning-info'
 
 export const API_URL = 'https://a.klaviyo.com/api'
 export const REVISION_DATE = KLAVIYO_REVISION_DATE
+export const MAX_EXTERNAL_ID_LENGTH = 255
 export const COUNTRY_CODES = [
   { label: 'AD - Andorra', value: 'AD' },
   { label: 'AE - United Arab Emirates', value: 'AE' },
diff --git a/packages/destination-actions/src/destinations/klaviyo/functions.ts b/packages/destination-actions/src/destinations/klaviyo/functions.ts
index bb0efcaf0c9..03f9f871c84 100644
--- a/packages/destination-actions/src/destinations/klaviyo/functions.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/functions.ts
@@ -10,7 +10,7 @@ import {
   ErrorCodes,
   StatsContext
 } from '@segment/actions-core'
-import { API_URL, REVISION_DATE } from './config'
+import { API_URL, REVISION_DATE, MAX_EXTERNAL_ID_LENGTH } from './config'
 import { Settings } from './generated-types'
 import {
   KlaviyoAPIError,
@@ -40,6 +40,18 @@ import { ActionDestinationErrorResponseType } from '@segment/actions-core/destin
 
 const phoneUtil = PhoneNumberUtil.getInstance()
 
+const EXTERNAL_ID_LENGTH_ERROR: ActionDestinationErrorResponseType = {
+  status: 400,
+  errortype: 'PAYLOAD_VALIDATION_FAILED',
+  errormessage: `Length of external_id must be no more than ${MAX_EXTERNAL_ID_LENGTH} characters.`
+}
+
+export function validateExternalId(externalId: string | undefined): void {
+  if (externalId && externalId.length > MAX_EXTERNAL_ID_LENGTH) {
+    throw new PayloadValidationError(EXTERNAL_ID_LENGTH_ERROR.errormessage)
+  }
+}
+
 export async function getListIdDynamicData(request: RequestClient): Promise<DynamicFieldResponse> {
   try {
     const result: ListIdResponse = await request(`${API_URL}/lists/`, {
@@ -99,6 +111,7 @@ export async function createProfile(
   phone_number: string | undefined,
   additionalAttributes: AdditionalAttributes
 ) {
+  validateExternalId(external_id)
   try {
     const profileData: ProfileData = {
       data: {
@@ -586,6 +599,11 @@ function validateAndConstructRemoveProfilePayloads(payload: RemoveProfilePayload
     return response
   }
 
+  if (external_id && external_id.length > MAX_EXTERNAL_ID_LENGTH) {
+    response.error = EXTERNAL_ID_LENGTH_ERROR
+    return response
+  }
+
   if (phone_number) {
     const validPhoneNumber = validateAndConvertPhoneNumber(phone_number, payload.country_code as string)
     if (!validPhoneNumber) {
@@ -616,6 +634,11 @@ function validateAndConstructProfilePayload(payload: AddProfileToListPayload): {
     return response
   }
 
+  if (external_id && external_id.length > MAX_EXTERNAL_ID_LENGTH) {
+    response.error = EXTERNAL_ID_LENGTH_ERROR
+    return response
+  }
+
   if (phone_number) {
     const validPhoneNumber = validateAndConvertPhoneNumber(phone_number, payload.country_code as string)
     if (!validPhoneNumber) {
@@ -756,20 +779,22 @@ function validateAndPreparePayloads(payloads: TrackEventPayload[], multiStatusRe
       return
     }
 
+    if (external_id && external_id.length > MAX_EXTERNAL_ID_LENGTH) {
+      multiStatusResponse.setErrorResponseAtIndex(originalBatchIndex, EXTERNAL_ID_LENGTH_ERROR)
+      return
+    }
+
     if (phone_number) {
-      // Validate and convert the phone number if present
       const validPhoneNumber = validateAndConvertPhoneNumber(phone_number, country_code as string)
-      // If the phone number is not valid, skip this payload
       if (!validPhoneNumber) {
         multiStatusResponse.setErrorResponseAtIndex(originalBatchIndex, {
           status: 400,
           errortype: 'PAYLOAD_VALIDATION_FAILED',
           errormessage: 'Phone number could not be converted to E.164 format.'
         })
-        return // Skip this payload
+        return
       }
 
-      // Update the payload's phone number with the validated format
       payload.profile.phone_number = validPhoneNumber
       delete payload?.profile?.country_code
     }
@@ -920,6 +945,11 @@ export function validateProfilePayload(payload: Payload): validateProfilePayload
     return response
   }
 
+  if (payload.external_id && payload.external_id.length > MAX_EXTERNAL_ID_LENGTH) {
+    response.error = EXTERNAL_ID_LENGTH_ERROR
+    return response
+  }
+
   if (payload.phone_number) {
     const validPhoneNumber = validateAndConvertPhoneNumber(payload.phone_number, payload.country_code as string)
     if (!validPhoneNumber) {
diff --git a/packages/destination-actions/src/destinations/klaviyo/orderCompleted/__tests__/index.test.ts b/packages/destination-actions/src/destinations/klaviyo/orderCompleted/__tests__/index.test.ts
index 5f1967d7b9c..3bf60152f51 100644
--- a/packages/destination-actions/src/destinations/klaviyo/orderCompleted/__tests__/index.test.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/orderCompleted/__tests__/index.test.ts
@@ -53,6 +53,23 @@ describe('Order Completed', () => {
     )
   })
 
+  it('should throw error if external_id exceeds 255 characters', async () => {
+    const event = createTestEvent({
+      type: 'track',
+      timestamp: '2022-01-01T00:00:00.000Z'
+    })
+
+    const mapping = {
+      profile: { external_id: 'a'.repeat(256) },
+      metric_name: 'Order Completed',
+      properties: { key: 'value' }
+    }
+
+    await expect(testDestination.testAction('orderCompleted', { event, mapping, settings })).rejects.toThrowError(
+      'Length of external_id must be no more than 255 characters.'
+    )
+  })
+
   it('should throw an error for invalid phone number format', async () => {
     const profile = { email: 'test@example.com', phone_number: 'invalid-phone-number', country_code: 'US' }
     const properties = { key: 'value' }
diff --git a/packages/destination-actions/src/destinations/klaviyo/orderCompleted/index.ts b/packages/destination-actions/src/destinations/klaviyo/orderCompleted/index.ts
index 3417988c41b..49d07a974ea 100644
--- a/packages/destination-actions/src/destinations/klaviyo/orderCompleted/index.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/orderCompleted/index.ts
@@ -5,7 +5,7 @@ import { PayloadValidationError, RequestClient } from '@segment/actions-core'
 import { API_URL } from '../config'
 import { EventData } from '../types'
 import { v4 as uuidv4 } from '@lukeed/uuid'
-import { processPhoneNumber } from '../functions'
+import { processPhoneNumber, validateExternalId } from '../functions'
 import { country_code } from '../properties'
 import dayjs from 'dayjs'
 
@@ -176,6 +176,7 @@ const action: ActionDefinition<Settings, Payload> = {
     if (!email && !phone_number && !external_id && !anonymous_id) {
       throw new PayloadValidationError('One of External ID, Anonymous ID, Phone Number or Email is required.')
     }
+    validateExternalId(external_id)
 
     const eventData = createEventData(payload)
 
diff --git a/packages/destination-actions/src/destinations/klaviyo/properties.ts b/packages/destination-actions/src/destinations/klaviyo/properties.ts
index c0ae56e66b1..74662e30ba7 100644
--- a/packages/destination-actions/src/destinations/klaviyo/properties.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/properties.ts
@@ -30,8 +30,10 @@ export const email: InputField = {
 
 export const external_id: InputField = {
   label: 'External ID',
-  description: `A unique identifier used by customers to associate Klaviyo profiles with profiles in an external system. One of External ID and Email required.`,
-  type: 'string'
+  description: `A unique identifier used by customers to associate Klaviyo profiles with profiles in an external system. One of External ID, Email or Phone Number is required. Must not exceed 255 characters.`,
+  type: 'string',
+  minimum: 0,
+  maximum: 255
 }
 
 export const enable_batching: InputField = {
diff --git a/packages/destination-actions/src/destinations/klaviyo/removeProfile/__tests__/index.test.ts b/packages/destination-actions/src/destinations/klaviyo/removeProfile/__tests__/index.test.ts
index 991263b9246..89661890630 100644
--- a/packages/destination-actions/src/destinations/klaviyo/removeProfile/__tests__/index.test.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/removeProfile/__tests__/index.test.ts
@@ -25,6 +25,22 @@ describe('Remove Profile', () => {
     )
   })
 
+  it('should throw error if external_id exceeds 255 characters', async () => {
+    const event = createTestEvent({
+      type: 'track',
+      properties: {}
+    })
+
+    const mapping = {
+      list_id: listId,
+      external_id: 'a'.repeat(256)
+    }
+
+    await expect(testDestination.testAction('removeProfile', { event, mapping, settings })).rejects.toThrowError(
+      'Length of external_id must be no more than 255 characters.'
+    )
+  })
+
   it('should throw an error for invalid phone number format', async () => {
     const event = createTestEvent({
       type: 'track',
diff --git a/packages/destination-actions/src/destinations/klaviyo/removeProfile/index.ts b/packages/destination-actions/src/destinations/klaviyo/removeProfile/index.ts
index 7aa007b937a..0c9eef95723 100644
--- a/packages/destination-actions/src/destinations/klaviyo/removeProfile/index.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/removeProfile/index.ts
@@ -6,6 +6,7 @@ import {
   getListIdDynamicData,
   getProfiles,
   processPhoneNumber,
+  validateExternalId,
   removeBulkProfilesFromList,
   removeProfileFromList
 } from '../functions'
@@ -67,6 +68,7 @@ const action: ActionDefinition<Settings, Payload> = {
     if (!email && !external_id && !phone_number) {
       throw new PayloadValidationError('One of External ID, Phone Number and Email is required.')
     }
+    validateExternalId(external_id)
     const profileIds = await getProfiles(
       request,
       email ? [email] : undefined,
diff --git a/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/__tests__/index.test.ts b/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/__tests__/index.test.ts
index 4201cfcf445..addc34f1246 100644
--- a/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/__tests__/index.test.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/__tests__/index.test.ts
@@ -1,8 +1,8 @@
 import nock from 'nock'
 import { createTestEvent, createTestIntegration } from '@segment/actions-core'
+import { AggregateAjvError } from '@segment/ajv-human-errors'
 import Definition from '../../index'
 import { API_URL } from '../../config'
-import { AggregateAjvError } from '@segment/ajv-human-errors'
 
 const testDestination = createTestIntegration(Definition)
 
@@ -25,6 +25,22 @@ describe('Remove List from Profile', () => {
     )
   })
 
+  it('should throw error if external_id exceeds 255 characters', async () => {
+    const event = createTestEvent({
+      type: 'track',
+      properties: {}
+    })
+
+    const mapping = {
+      list_id: listId,
+      external_id: 'a'.repeat(256)
+    }
+
+    await expect(
+      testDestination.testAction('removeProfileFromList', { event, mapping, settings })
+    ).rejects.toThrowError(AggregateAjvError)
+  })
+
   it('should throw an error for invalid phone number format', async () => {
     const event = createTestEvent({
       type: 'track',
diff --git a/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/generated-types.ts b/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/generated-types.ts
index 055596f972b..dd062d87a2f 100644
--- a/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/generated-types.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/generated-types.ts
@@ -6,7 +6,7 @@ export interface Payload {
    */
   email?: string
   /**
-   * A unique identifier used by customers to associate Klaviyo profiles with profiles in an external system. One of External ID and Email required.
+   * A unique identifier used by customers to associate Klaviyo profiles with profiles in an external system. One of External ID, Email or Phone Number is required. Must not exceed 255 characters.
    */
   external_id?: string
   /**
diff --git a/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/index.ts b/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/index.ts
index be73107fa09..00dc73cd1d0 100644
--- a/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/index.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/removeProfileFromList/index.ts
@@ -2,7 +2,13 @@ import { ActionDefinition, PayloadValidationError } from '@segment/actions-core'
 import type { Settings } from '../generated-types'
 import { Payload } from './generated-types'
 
-import { getProfiles, processPhoneNumber, removeBulkProfilesFromList, removeProfileFromList } from '../functions'
+import {
+  getProfiles,
+  processPhoneNumber,
+  validateExternalId,
+  removeBulkProfilesFromList,
+  removeProfileFromList
+} from '../functions'
 import { email, list_id, external_id, enable_batching, phone_number, country_code, batch_size } from '../properties'
 
 const action: ActionDefinition<Settings, Payload> = {
@@ -33,6 +39,7 @@ const action: ActionDefinition<Settings, Payload> = {
     if (!email && !external_id && !phone_number) {
       throw new PayloadValidationError('One of External ID, Phone Number and Email is required.')
     }
+    validateExternalId(external_id)
     const profileIds = await getProfiles(
       request,
       email ? [email] : undefined,
diff --git a/packages/destination-actions/src/destinations/klaviyo/trackEvent/__tests__/index.test.ts b/packages/destination-actions/src/destinations/klaviyo/trackEvent/__tests__/index.test.ts
index 1a319310a53..3c7e7cf64bb 100644
--- a/packages/destination-actions/src/destinations/klaviyo/trackEvent/__tests__/index.test.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/trackEvent/__tests__/index.test.ts
@@ -28,6 +28,23 @@ describe('Track Event', () => {
     )
   })
 
+  it('should throw error if external_id exceeds 255 characters', async () => {
+    const event = createTestEvent({
+      type: 'track',
+      timestamp: '2022-01-01T00:00:00.000Z'
+    })
+
+    const mapping = {
+      profile: { external_id: 'a'.repeat(256) },
+      metric_name: 'Test Event',
+      properties: {}
+    }
+
+    await expect(testDestination.testAction('trackEvent', { event, mapping, settings })).rejects.toThrowError(
+      'Length of external_id must be no more than 255 characters.'
+    )
+  })
+
   it('should throw an error for invalid phone number format', async () => {
     const profile = { email: 'test@example.com', phone_number: 'invalid-phone-number', country_code: 'US' }
     const properties = { key: 'value' }
diff --git a/packages/destination-actions/src/destinations/klaviyo/trackEvent/index.ts b/packages/destination-actions/src/destinations/klaviyo/trackEvent/index.ts
index 1a8fc078057..3e3c0cb5b42 100644
--- a/packages/destination-actions/src/destinations/klaviyo/trackEvent/index.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/trackEvent/index.ts
@@ -4,7 +4,7 @@ import type { Payload } from './generated-types'
 import { PayloadValidationError } from '@segment/actions-core'
 import { API_URL } from '../config'
 import { batch_size, enable_batching, country_code } from '../properties'
-import { processPhoneNumber, sendBatchedTrackEvent } from '../functions'
+import { processPhoneNumber, sendBatchedTrackEvent, validateExternalId } from '../functions'
 import dayjs from '../../../lib/dayjs'
 
 const action: ActionDefinition<Settings, Payload> = {
@@ -106,6 +106,7 @@ const action: ActionDefinition<Settings, Payload> = {
     if (!email && !phone_number && !external_id && !anonymous_id) {
       throw new PayloadValidationError('One of External ID, Anonymous ID, Phone Number or Email is required.')
     }
+    validateExternalId(external_id)
     const eventData = {
       data: {
         type: 'event',
diff --git a/packages/destination-actions/src/destinations/klaviyo/upsertProfile/__tests__/index.test.ts b/packages/destination-actions/src/destinations/klaviyo/upsertProfile/__tests__/index.test.ts
index 6050e086133..d787f59fb7e 100644
--- a/packages/destination-actions/src/destinations/klaviyo/upsertProfile/__tests__/index.test.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/upsertProfile/__tests__/index.test.ts
@@ -50,6 +50,44 @@ describe('Upsert Profile', () => {
     )
   })
 
+  it('should throw error if external_id exceeds 255 characters', async () => {
+    const longExternalId = 'a'.repeat(256)
+    const event = createTestEvent({
+      type: 'identify',
+      traits: {
+        email: 'test@example.com'
+      }
+    })
+
+    const mapping = {
+      external_id: longExternalId,
+      email: { '@path': '$.traits.email' }
+    }
+
+    await expect(testDestination.testAction('upsertProfile', { event, settings, mapping })).rejects.toThrowError(
+      'Length of external_id must be no more than 255 characters.'
+    )
+  })
+
+  it('should not throw error if external_id is exactly 255 characters', async () => {
+    const exactExternalId = 'a'.repeat(255)
+    const event = createTestEvent({
+      type: 'identify',
+      traits: {
+        email: 'test@example.com'
+      }
+    })
+
+    const mapping = {
+      external_id: exactExternalId,
+      email: { '@path': '$.traits.email' }
+    }
+
+    nock(`${API_URL}`).post('/profiles/').reply(200, {})
+
+    await expect(testDestination.testAction('upsertProfile', { event, settings, mapping })).resolves.not.toThrowError()
+  })
+
   it('should throw an error for invalid phone number format in perform', async () => {
     const event = createTestEvent({
       type: 'identify',
diff --git a/packages/destination-actions/src/destinations/klaviyo/upsertProfile/index.ts b/packages/destination-actions/src/destinations/klaviyo/upsertProfile/index.ts
index a82e0ab6c8a..31a18ebff0c 100644
--- a/packages/destination-actions/src/destinations/klaviyo/upsertProfile/index.ts
+++ b/packages/destination-actions/src/destinations/klaviyo/upsertProfile/index.ts
@@ -13,6 +13,7 @@ import {
   getList,
   createList,
   processPhoneNumber,
+  validateExternalId,
   validateProfilePayload,
   updateMultiStatusWithSuccessData,
   updateMultiStatusWithKlaviyoErrors
@@ -260,6 +261,8 @@ const action: ActionDefinition<Settings, Payload> = {
       throw new PayloadValidationError('One of External ID, Phone Number and Email is required.')
     }
 
+    validateExternalId(external_id)
+
     const profileData: ProfileData = {
       data: {
         type: 'profile',