diff --git a/src/errors.ts b/src/errors.ts
index 02e444b..8b6ced2 100644
--- a/src/errors.ts
+++ b/src/errors.ts
@@ -299,6 +299,22 @@ export class ValidationError extends StellarSplitError {
   }
 }
 
+/** Thrown when split recipient ratios do not sum to exactly 1.0. */
+export class SplitRatioSumError extends StellarSplitError {
+  readonly actualSum: number;
+
+  constructor(actualSum: number, tolerance: number) {
+    super(
+      `Split ratios sum to ${actualSum} but must equal 1.0 (tolerance: ±${tolerance})`,
+      "SPLIT_RATIO_SUM_ERROR",
+      { actualSum, tolerance },
+    );
+    this.name = "SplitRatioSumError";
+    this.actualSum = actualSum;
+    Object.setPrototypeOf(this, new.target.prototype);
+  }
+}
+
 /** Thrown when a plugin with the same name is already registered. */
 export class PluginAlreadyRegisteredError extends StellarSplitError {
   readonly pluginName: string;
diff --git a/src/index.ts b/src/index.ts
index ebf7e30..f5c0a11 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -57,6 +57,7 @@ export {
   ContractError,
   CircuitOpenError,
   ValidationError,
+  SplitRatioSumError,
   PluginAlreadyRegisteredError,
   InvalidBatchSizeError,
   InvoiceNotReleasedError,
diff --git a/src/payments/splitExecutor.ts b/src/payments/splitExecutor.ts
index d26b839..d116685 100644
--- a/src/payments/splitExecutor.ts
+++ b/src/payments/splitExecutor.ts
@@ -12,17 +12,23 @@
 
 import { checkSubentryCapacity, SubentryCapacityGuardError } from "../account/subentryGuard.js";
 import type { SubentryCapacityResult } from "../types.js";
+import { SplitRatioSumError } from "../errors.js";
 
 // ---------------------------------------------------------------------------
 // Public types
 // ---------------------------------------------------------------------------
 
+/** Tolerance for floating-point ratio-sum comparison. Exported for callers. */
+export const SPLIT_RATIO_TOLERANCE = 1e-9;
+
 /** A single recipient leg in a split payment. */
 export interface SplitRecipient {
   /** Stellar G… address of the recipient. */
   address: string;
   /** Amount to send in stroops. */
   amount: bigint;
+  /** Optional share ratio (0.0–1.0). When provided, splitExecutor validates that all ratios sum to 1.0. */
+  ratio?: number;
   /**
    * Number of new subentry slots this recipient will consume as a result of
    * this operation (e.g., 1 for a new trustline, 1 for a new data entry).
@@ -63,6 +69,23 @@ export interface SplitExecutionResult {
 // splitExecutor
 // ---------------------------------------------------------------------------
 
+/**
+ * Validates that any provided recipient ratios sum to 1.0 within tolerance.
+ * @throws {SplitRatioSumError} When ratios are provided and do not sum to 1.0.
+ */
+function validateRecipientRatios(recipients: SplitRecipient[]): void {
+  const ratios = recipients
+    .map((r) => r.ratio)
+    .filter((r): r is number => r !== undefined);
+
+  if (ratios.length === 0) return;
+
+  const sum = ratios.reduce((acc, r) => acc + r, 0);
+  if (Math.abs(sum - 1.0) > SPLIT_RATIO_TOLERANCE) {
+    throw new SplitRatioSumError(sum, SPLIT_RATIO_TOLERANCE);
+  }
+}
+
 /**
  * Executes a multi-recipient split payment after running subentry capacity
  * pre-flight checks for each recipient.
@@ -74,6 +97,7 @@ export interface SplitExecutionResult {
  *
  * @throws {SubentryCapacityGuardError} When any recipient's account cannot
  *   accommodate the required subentry slots and `skipCapacityCheck` is not set.
+ * @throws {SplitRatioSumError} When recipient ratios are provided and do not sum to 1.0.
  *
  * @example
  * ```ts
@@ -99,6 +123,9 @@ export async function splitExecutor(
     horizonUrl = "https://horizon.stellar.org",
   } = options;
 
+  // Ratio validation (issue #778)
+  validateRecipientRatios(recipients);
+
   const capacityChecks: Record<string, SubentryCapacityResult> = {};
 
   if (!skipCapacityCheck) {
diff --git a/test/splitExecutor.test.ts b/test/splitExecutor.test.ts
new file mode 100644
index 0000000..a5e6f03
--- /dev/null
+++ b/test/splitExecutor.test.ts
@@ -0,0 +1,92 @@
+import { describe, it, expect, vi } from "vitest";
+import {
+  splitExecutor,
+  SPLIT_RATIO_TOLERANCE,
+  type SplitRecipient,
+} from "../src/payments/splitExecutor.js";
+import { SplitRatioSumError } from "../src/errors.js";
+
+// Mock the subentry guard so tests don't hit Horizon
+vi.mock("../src/account/subentryGuard.js", () => ({
+  checkSubentryCapacity: vi.fn().mockResolvedValue({ sufficient: true, availableSlots: 10 }),
+  SubentryCapacityGuardError: class extends Error {},
+}));
+
+describe("splitExecutor", () => {
+  it("proceeds when no ratios are provided", async () => {
+    const recipients: SplitRecipient[] = [
+      { address: "GABC...", amount: 5_000_000n },
+      { address: "GDEF...", amount: 5_000_000n },
+    ];
+    const result = await splitExecutor(recipients, { skipCapacityCheck: true });
+    expect(result.success).toBe(true);
+  });
+
+  it("proceeds when ratios sum to 1.0 within tolerance", async () => {
+    const recipients: SplitRecipient[] = [
+      { address: "GABC...", amount: 4_000_000n, ratio: 0.4 },
+      { address: "GDEF...", amount: 3_000_000n, ratio: 0.3 },
+      { address: "GHIJ...", amount: 3_000_000n, ratio: 0.3 },
+    ];
+    const result = await splitExecutor(recipients, { skipCapacityCheck: true });
+    expect(result.success).toBe(true);
+  });
+
+  it("throws SplitRatioSumError when ratios sum to less than 1.0", async () => {
+    const recipients: SplitRecipient[] = [
+      { address: "GABC...", amount: 3_000_000n, ratio: 0.3 },
+      { address: "GDEF...", amount: 3_000_000n, ratio: 0.3 },
+    ];
+    await expect(splitExecutor(recipients, { skipCapacityCheck: true })).rejects.toThrow(
+      SplitRatioSumError,
+    );
+  });
+
+  it("throws SplitRatioSumError when ratios sum to more than 1.0", async () => {
+    const recipients: SplitRecipient[] = [
+      { address: "GABC...", amount: 6_000_000n, ratio: 0.6 },
+      { address: "GDEF...", amount: 5_000_000n, ratio: 0.5 },
+    ];
+    await expect(splitExecutor(recipients, { skipCapacityCheck: true })).rejects.toThrow(
+      SplitRatioSumError,
+    );
+  });
+
+  it("includes actualSum in the thrown SplitRatioSumError", async () => {
+    const recipients: SplitRecipient[] = [
+      { address: "GABC...", amount: 3_000_000n, ratio: 0.3 },
+      { address: "GDEF...", amount: 3_000_000n, ratio: 0.3 },
+    ];
+    try {
+      await splitExecutor(recipients, { skipCapacityCheck: true });
+      expect.fail("Should have thrown");
+    } catch (err) {
+      expect(err).toBeInstanceOf(SplitRatioSumError);
+      expect((err as SplitRatioSumError).actualSum).toBeCloseTo(0.6, 10);
+    }
+  });
+
+  it("tolerates floating-point rounding within SPLIT_RATIO_TOLERANCE", async () => {
+    const recipients: SplitRecipient[] = [
+      { address: "GABC...", amount: 3_333_333n, ratio: 0.3333333333 },
+      { address: "GDEF...", amount: 3_333_333n, ratio: 0.3333333333 },
+      { address: "GHIJ...", amount: 3_333_334n, ratio: 0.3333333334 },
+    ];
+    const result = await splitExecutor(recipients, { skipCapacityCheck: true });
+    expect(result.success).toBe(true);
+  });
+
+  it("rejects when rounding exceeds SPLIT_RATIO_TOLERANCE", async () => {
+    const recipients: SplitRecipient[] = [
+      { address: "GABC...", amount: 5_000_000n, ratio: 0.5 },
+      { address: "GDEF...", amount: 5_000_000n, ratio: 0.500000002 },
+    ];
+    await expect(splitExecutor(recipients, { skipCapacityCheck: true })).rejects.toThrow(
+      SplitRatioSumError,
+    );
+  });
+
+  it("exports SPLIT_RATIO_TOLERANCE as 1e-9", () => {
+    expect(SPLIT_RATIO_TOLERANCE).toBe(1e-9);
+  });
+});