diff --git a/docs/STRUCTURED_JSON_LOGGING.md b/docs/STRUCTURED_JSON_LOGGING.md
new file mode 100644
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--- /dev/null
+++ b/docs/STRUCTURED_JSON_LOGGING.md
@@ -0,0 +1,45 @@
+# 📊 Structured JSON Logging Configuration
+
+This document details the machine-readable JSON logging format supported across NotifyChain (Issue #685).
+
+---
+
+## 1. Enabling Structured JSON Logging
+
+Structured JSON logging can be activated via environment variables:
+
+```bash
+# Enable explicitly
+STRUCTURED_LOGGING=true
+
+# Alternative format selector
+LOG_FORMAT=json
+```
+
+*(Note: Structured JSON format is automatically active by default in `NODE_ENV=production`)*.
+
+---
+
+## 2. Standard JSON Schema
+
+Each log record is emitted as a single newline-delimited JSON (NDJSON) string:
+
+```json
+{
+  "timestamp": "2026-08-29T12:00:00.000Z",
+  "level": "info",
+  "message": "Notification dispatched to Discord endpoint",
+  "service": "notify-chain",
+  "environment": "production",
+  "requestId": "req-9b1deb4d-3b7d-4bad-9bdd-2b0d7b3dcb6d",
+  "durationMs": 45.2,
+  "provider": "Discord",
+  "notificationId": "notif-12345"
+}
+```
+
+---
+
+## 3. Redaction & Invariant Security
+
+All metadata fields and nested error structures are continuously filtered through the centralized redaction engine (`listener/src/utils/redact.ts`) before serialization, ensuring private keys, webhook tokens, and credentials are never ingested into log aggregators.
diff --git a/listener/src/utils/json-logger-formatter.test.ts b/listener/src/utils/json-logger-formatter.test.ts
new file mode 100644
index 00000000..ba359948
--- /dev/null
+++ b/listener/src/utils/json-logger-formatter.test.ts
@@ -0,0 +1,72 @@
+import {
+  createStructuredJsonFormat,
+  isJsonLoggingEnabled,
+} from './json-logger-formatter';
+
+describe('Structured JSON Logging Option (Issue #685)', () => {
+  describe('isJsonLoggingEnabled', () => {
+    test('enables when STRUCTURED_LOGGING=true', () => {
+      expect(isJsonLoggingEnabled({ STRUCTURED_LOGGING: 'true' })).toBe(true);
+    });
+
+    test('enables when LOG_FORMAT=json', () => {
+      expect(isJsonLoggingEnabled({ LOG_FORMAT: 'json' })).toBe(true);
+    });
+
+    test('enables by default in production unless explicitly disabled', () => {
+      expect(isJsonLoggingEnabled({ NODE_ENV: 'production' })).toBe(true);
+      expect(isJsonLoggingEnabled({ NODE_ENV: 'production', STRUCTURED_LOGGING: 'false' })).toBe(false);
+    });
+  });
+
+  describe('createStructuredJsonFormat', () => {
+    test('formats log info into standard single-line JSON', () => {
+      const formatter = createStructuredJsonFormat({
+        serviceName: 'notify-chain-listener',
+        environment: 'staging',
+      });
+
+      const logInfo = {
+        level: 'info',
+        message: 'Notification delivered successfully',
+        timestamp: '2026-08-29T12:00:00.000Z',
+        requestId: 'req-456',
+        durationMs: 42.5,
+        eventId: 'evt-001',
+      };
+
+      const result = (formatter.transform(logInfo as any) as any)[Symbol.for('message')];
+      const parsed = JSON.parse(result);
+
+      expect(parsed.level).toBe('info');
+      expect(parsed.service).toBe('notify-chain-listener');
+      expect(parsed.environment).toBe('staging');
+      expect(parsed.requestId).toBe('req-456');
+      expect(parsed.durationMs).toBe(42.5);
+      expect(parsed.eventId).toBe('evt-001');
+    });
+
+    test('strictly redacts sensitive secrets in structured JSON logs', () => {
+      const secret = 'SCZANGBA5YHTNYVVV4C3U252E2B6P6IRKD45DCAHSKV2U2B6P6IRKD45';
+      const formatter = createStructuredJsonFormat();
+
+      const logInfo = {
+        level: 'error',
+        message: `Failed sending to endpoint using ${secret}`,
+        timestamp: '2026-08-29T12:00:00.000Z',
+        apiKey: 'super-secret-key-12345',
+        error: {
+          name: 'AuthError',
+          message: `Invalid token ${secret}`,
+        },
+      };
+
+      const result = (formatter.transform(logInfo as any) as any)[Symbol.for('message')];
+      const parsed = JSON.parse(result);
+
+      expect(parsed.apiKey).toBe('[REDACTED]');
+      expect(parsed.message).not.toContain(secret);
+      expect(parsed.error.message).not.toContain(secret);
+    });
+  });
+});
diff --git a/listener/src/utils/json-logger-formatter.ts b/listener/src/utils/json-logger-formatter.ts
new file mode 100644
index 00000000..6451eaf8
--- /dev/null
+++ b/listener/src/utils/json-logger-formatter.ts
@@ -0,0 +1,71 @@
+/**
+ * Structured JSON Log Formatter (Issue #685)
+ *
+ * Produces deterministic, machine-readable JSON logs for log aggregators
+ * (Datadog, CloudWatch, Grafana Loki, Elasticsearch) with automated secret redaction.
+ */
+
+import winston from 'winston';
+import { redactSensitiveData, redactString } from './redact';
+
+export interface StructuredLogRecord {
+  timestamp: string;
+  level: string;
+  message: string;
+  service: string;
+  environment: string;
+  requestId?: string;
+  durationMs?: number;
+  error?: Record<string, unknown> | string;
+  [key: string]: unknown;
+}
+
+export interface JsonFormatterOptions {
+  serviceName?: string;
+  environment?: string;
+}
+
+/**
+ * Builds a Winston format function that converts log entries into normalized JSON.
+ */
+export function createStructuredJsonFormat(options: JsonFormatterOptions = {}) {
+  const service = options.serviceName || process.env.SERVICE_NAME || 'notify-chain';
+  const environment = options.environment || process.env.NODE_ENV || 'development';
+
+  return winston.format.printf((info) => {
+    const { level, message, timestamp, requestId, durationMs, error, ...rest } = info;
+
+    // Sanitize message string
+    const sanitizedMessage = typeof message === 'string' ? redactString(message) : String(message);
+
+    // Sanitize extra metadata
+    const sanitizedMetadata = redactSensitiveData(rest) as Record<string, unknown>;
+
+    const record: StructuredLogRecord = {
+      timestamp: (timestamp as string) || new Date().toISOString(),
+      level,
+      message: sanitizedMessage,
+      service,
+      environment,
+      ...(requestId ? { requestId: String(requestId) } : {}),
+      ...(durationMs !== undefined ? { durationMs: Number(durationMs) } : {}),
+      ...(error ? { error: redactSensitiveData(error) as Record<string, unknown> } : {}),
+      ...sanitizedMetadata,
+    };
+
+    return JSON.stringify(record);
+  });
+}
+
+/**
+ * Determines whether structured JSON logging should be enabled.
+ */
+export function isJsonLoggingEnabled(env: Record<string, string | undefined> = process.env): boolean {
+  if (env.STRUCTURED_LOGGING === 'true' || env.LOG_FORMAT === 'json') {
+    return true;
+  }
+  if (env.NODE_ENV === 'production' && env.STRUCTURED_LOGGING !== 'false') {
+    return true;
+  }
+  return false;
+}