diff --git a/src/auditLog.test.ts b/src/auditLog.test.ts
new file mode 100644
index 0000000..0d23d7e
--- /dev/null
+++ b/src/auditLog.test.ts
@@ -0,0 +1,126 @@
+import { describe, it, beforeEach } from "node:test";
+import assert from "node:assert";
+import { AuditLog, redactSecrets } from "./auditLog.js";
+
+await describe("AuditLog", async () => {
+  let auditLog: AuditLog;
+
+  await Promise.resolve(beforeEach(() => {
+    auditLog = new AuditLog();
+  }));
+
+  await it("records an entry for a mutation", () => {
+    const entry = auditLog.append({
+      actor: "tenant-1",
+      action: "charge.create",
+      targetId: "ch_123",
+      targetType: "charge",
+      after: { amount: 100, currency: "USD" },
+    });
+    assert.match(entry.id, /^audit_/);
+    assert.strictEqual(entry.actor, "tenant-1");
+    assert.strictEqual(entry.action, "charge.create");
+    assert.strictEqual(entry.targetId, "ch_123");
+    assert.strictEqual(auditLog.size(), 1);
+  });
+
+  await it("does not record an entry for a read", () => {
+    assert.strictEqual(auditLog.size(), 0);
+  });
+
+  await it("queries by target id", () => {
+    auditLog.append({
+      actor: "tenant-1",
+      action: "charge.create",
+      targetId: "ch_123",
+      targetType: "charge",
+      after: { amount: 100 },
+    });
+    auditLog.append({
+      actor: "tenant-1",
+      action: "charge.create",
+      targetId: "ch_456",
+      targetType: "charge",
+      after: { amount: 200 },
+    });
+    const entries = auditLog.queryByTarget("ch_123");
+    assert.strictEqual(entries.length, 1);
+    assert.strictEqual(entries[0].targetId, "ch_123");
+  });
+
+  await it("queries by actor", () => {
+    auditLog.append({
+      actor: "tenant-a",
+      action: "charge.create",
+      targetId: "ch_1",
+      targetType: "charge",
+      after: { amount: 100 },
+    });
+    auditLog.append({
+      actor: "tenant-b",
+      action: "charge.create",
+      targetId: "ch_2",
+      targetType: "charge",
+      after: { amount: 200 },
+    });
+    const entries = auditLog.queryByActor("tenant-a");
+    assert.strictEqual(entries.length, 1);
+    assert.strictEqual(entries[0].actor, "tenant-a");
+  });
+
+  await it("supports pagination", () => {
+    for (let i = 0; i < 5; i++) {
+      auditLog.append({
+        actor: "tenant-1",
+        action: "charge.create",
+        targetId: `ch_${i}`,
+        targetType: "charge",
+        after: { amount: i },
+      });
+    }
+    const entries = auditLog.queryByActor("tenant-1", 2, 1);
+    assert.strictEqual(entries.length, 2);
+    assert.strictEqual(entries[0].targetId, "ch_1");
+    assert.strictEqual(entries[1].targetId, "ch_2");
+  });
+
+  await it("never redacts non-sensitive fields", () => {
+    const payload = { amount: 100, currency: "USD" };
+    const redacted = redactSecrets(payload);
+    assert.strictEqual(redacted.amount, 100);
+    assert.strictEqual(redacted.currency, "USD");
+  });
+
+  await it("redacts secrets from payloads", () => {
+    const payload = {
+      amount: 100,
+      apiToken: "super-secret",
+      password: "hunter2",
+      nested: { ssn: "123-45-6789", name: "Alice" },
+    };
+    const redacted = redactSecrets(payload);
+    assert.strictEqual(redacted.apiToken, "[REDACTED]");
+    assert.strictEqual(redacted.password, "[REDACTED]");
+    assert.strictEqual((redacted.nested as Record<string, unknown>).ssn, "[REDACTED]");
+    assert.strictEqual((redacted.nested as Record<string, unknown>).name, "Alice");
+  });
+
+  await it("entries are ordered by timestamp", () => {
+    auditLog.append({
+      actor: "tenant-1",
+      action: "charge.create",
+      targetId: "ch_1",
+      targetType: "charge",
+      after: { amount: 100 },
+    });
+    auditLog.append({
+      actor: "tenant-1",
+      action: "charge.create",
+      targetId: "ch_2",
+      targetType: "charge",
+      after: { amount: 200 },
+    });
+    const entries = auditLog.queryByActor("tenant-1");
+    assert.ok(entries[0].timestamp <= entries[1].timestamp);
+  });
+});
diff --git a/src/auditLog.ts b/src/auditLog.ts
new file mode 100644
index 0000000..366e43c
--- /dev/null
+++ b/src/auditLog.ts
@@ -0,0 +1,82 @@
+import { randomUUID } from "node:crypto";
+
+export type AuditAction = "charge.create" | "charge.update" | "charge.refund";
+
+export interface AuditEntry {
+  id: string;
+  actor: string; // tenantId or apiKey prefix
+  action: AuditAction;
+  targetId: string;
+  targetType: string;
+  before?: Record<string, unknown>;
+  after?: Record<string, unknown>;
+  timestamp: number;
+  requestId?: string;
+}
+
+/**
+ * Append-only, immutable audit log for state-changing payments operations.
+ * Entries are never updated or deleted.
+ */
+export class AuditLog {
+  private readonly entries: AuditEntry[] = [];
+
+  append(entry: Omit<AuditEntry, "id" | "timestamp">): AuditEntry {
+    const fullEntry: AuditEntry = {
+      ...entry,
+      id: `audit_${randomUUID()}`,
+      timestamp: Date.now(),
+    };
+    this.entries.push(fullEntry);
+    return fullEntry;
+  }
+
+  /** Query by target id, ordered by timestamp ascending. */
+  queryByTarget(targetId: string, limit?: number, offset?: number): AuditEntry[] {
+    const matches = this.entries.filter((e) => e.targetId === targetId);
+    return this.paginate(matches, limit, offset);
+  }
+
+  /** Query by actor, ordered by timestamp ascending. */
+  queryByActor(actor: string, limit?: number, offset?: number): AuditEntry[] {
+    const matches = this.entries.filter((e) => e.actor === actor);
+    return this.paginate(matches, limit, offset);
+  }
+
+  /** Total count of entries. */
+  size(): number {
+    return this.entries.length;
+  }
+
+  /** Clear all entries (test helper only). */
+  clear(): void {
+    this.entries.length = 0;
+  }
+
+  private paginate(items: AuditEntry[], limit?: number, offset?: number): AuditEntry[] {
+    const start = offset ?? 0;
+    const end = limit === undefined ? undefined : start + limit;
+    return items.slice(start, end);
+  }
+}
+
+/**
+ * Redacts sensitive fields from a payload before recording in the audit log.
+ * Removes any key that looks like a secret, token, password, or PII.
+ */
+export function redactSecrets(
+  payload: Record<string, unknown>
+): Record<string, unknown> {
+  const sensitiveKeys = /token|secret|password|ssn|sin|pin|cvv|authorization|auth/i;
+  const redacted: Record<string, unknown> = {};
+  for (const [key, value] of Object.entries(payload)) {
+    if (sensitiveKeys.test(key)) {
+      redacted[key] = "[REDACTED]";
+    } else if (typeof value === "object" && value !== null && !Array.isArray(value)) {
+      redacted[key] = redactSecrets(value as Record<string, unknown>);
+    } else {
+      redacted[key] = value;
+    }
+  }
+  return redacted;
+}
diff --git a/src/charges.ts b/src/charges.ts
index 0573915..f403904 100644
--- a/src/charges.ts
+++ b/src/charges.ts
@@ -1,4 +1,5 @@
 import { createHash, randomUUID } from "node:crypto";
+import { AuditLog, redactSecrets } from "./auditLog.js";
 
 export const CHARGE_KEY_TTL_MS = 24 * 60 * 60 * 1000;
 export const MAX_IDEMPOTENCY_KEY_LENGTH = 255;
@@ -139,6 +140,11 @@ export class InMemoryChargeIdempotencyStore implements ChargeIdempotencyStore {
 
 export class InMemoryChargeStore {
   private readonly charges: Charge[] = [];
+  private readonly auditLog?: AuditLog;
+
+  constructor(auditLog?: AuditLog) {
+    this.auditLog = auditLog;
+  }
 
   create(tenantId: string, input: ChargeInput, now = Date.now()): Charge {
     const charge: Charge = {
@@ -149,6 +155,13 @@ export class InMemoryChargeStore {
       status: "succeeded",
     };
     this.charges.push(charge);
+    this.auditLog?.append({
+      actor: tenantId,
+      action: "charge.create",
+      targetId: charge.id,
+      targetType: "charge",
+      after: redactSecrets(charge as unknown as Record<string, unknown>),
+    });
     return cloneCharge(charge);
   }
 
diff --git a/src/routes/charges.ts b/src/routes/charges.ts
index 801a004..c9fc764 100644
--- a/src/routes/charges.ts
+++ b/src/routes/charges.ts
@@ -16,17 +16,26 @@ import {
   paymentValidationError,
 } from "../errors/domainError.js";
 import { paymentErrorHandler } from "../middleware/paymentErrorHandler.js";
+import { parseIntParam } from "../queryParams.js";
+import { AuditLog } from "../auditLog.js";
 
 export type ChargesRouterOptions = {
   idempotencyStore?: InMemoryChargeIdempotencyStore;
   chargeStore?: InMemoryChargeStore;
+  auditLog?: AuditLog;
 };
 
+function firstQueryString(value: unknown): string {
+  const raw = Array.isArray(value) ? value[0] : value;
+  return typeof raw === "string" ? raw : "";
+}
+
 export function createChargesRouter(options: ChargesRouterOptions = {}): Router {
   const router = Router();
+  const auditLog = options.auditLog ?? new AuditLog();
   const idempotencyStore =
     options.idempotencyStore ?? new InMemoryChargeIdempotencyStore();
-  const chargeStore = options.chargeStore ?? new InMemoryChargeStore();
+  const chargeStore = options.chargeStore ?? new InMemoryChargeStore(auditLog);
 
   router.post("/api/v1/charges", (req: Request, res: Response, next: NextFunction) => {
     const requestId = paymentRequestId(req);
@@ -114,6 +123,38 @@ export function createChargesRouter(options: ChargesRouterOptions = {}): Router
     }
   );
 
+  router.get("/api/v1/charges/:chargeId/audit", (req: Request, res: Response) => {
+    const chargeId = String(req.params.chargeId);
+    const limit = parseIntParam(req.query.limit, {
+      defaultValue: 100,
+      min: 1,
+      max: 500,
+    });
+    const offset = parseIntParam(req.query.offset, {
+      defaultValue: 0,
+      min: 0,
+      max: 10_000,
+    });
+    const entries = auditLog.queryByTarget(chargeId, limit, offset);
+    res.status(200).json({ entries, requestId: paymentRequestId(req) });
+  });
+
+  router.get("/api/v1/audit", (req: Request, res: Response) => {
+    const actor = firstQueryString(req.query.actor);
+    const limit = parseIntParam(req.query.limit, {
+      defaultValue: 100,
+      min: 1,
+      max: 500,
+    });
+    const offset = parseIntParam(req.query.offset, {
+      defaultValue: 0,
+      min: 0,
+      max: 10_000,
+    });
+    const entries = auditLog.queryByActor(actor, limit, offset);
+    res.status(200).json({ entries, requestId: paymentRequestId(req) });
+  });
+
   router.use(paymentErrorHandler);
 
   return router;