diff --git a/apps/api/src/isms/isms-document-control.service.spec.ts b/apps/api/src/isms/isms-document-control.service.spec.ts
index c91cb7c11d..b209192048 100644
--- a/apps/api/src/isms/isms-document-control.service.spec.ts
+++ b/apps/api/src/isms/isms-document-control.service.spec.ts
@@ -14,6 +14,8 @@ jest.mock('@db', () => {
       createMany: jest.fn(),
       deleteMany: jest.fn(),
     },
+    // Advisory lock taken by invalidateApprovalIfNeeded (serializes vs approve).
+    $executeRaw: jest.fn(),
     // Run the callback with the same mock as the transaction client.
     $transaction: jest.fn((cb: (tx: unknown) => unknown) => cb(db)),
   };
diff --git a/apps/api/src/isms/isms-narrative.service.spec.ts b/apps/api/src/isms/isms-narrative.service.spec.ts
index 2770acd489..c1deacca14 100644
--- a/apps/api/src/isms/isms-narrative.service.spec.ts
+++ b/apps/api/src/isms/isms-narrative.service.spec.ts
@@ -9,6 +9,8 @@ jest.mock('@db', () => {
       findUnique: jest.fn(),
       update: jest.fn(),
     },
+    // Advisory lock taken by invalidateApprovalIfNeeded (serializes vs approve).
+    $executeRaw: jest.fn(),
     // Run the callback with the same mock as the transaction client.
     $transaction: jest.fn((cb: (tx: unknown) => unknown) => cb(db)),
   };
diff --git a/apps/api/src/isms/utils/approval.spec.ts b/apps/api/src/isms/utils/approval.spec.ts
new file mode 100644
index 0000000000..9ca4a07801
--- /dev/null
+++ b/apps/api/src/isms/utils/approval.spec.ts
@@ -0,0 +1,62 @@
+import { invalidateApprovalIfNeeded } from './approval';
+
+/**
+ * invalidateApprovalIfNeeded is the central serialization point: every ISMS
+ * content-mutation path calls it, and it must take the per-document advisory lock
+ * (so edits serialize against approve()) BEFORE reading/altering status.
+ */
+function makeTx() {
+  return {
+    $executeRaw: jest.fn().mockResolvedValue(0),
+    ismsDocument: {
+      findUnique: jest.fn(),
+      update: jest.fn().mockResolvedValue({}),
+    },
+  };
+}
+
+describe('invalidateApprovalIfNeeded', () => {
+  it('acquires the per-document advisory lock before reading status', async () => {
+    const tx = makeTx();
+    tx.ismsDocument.findUnique.mockResolvedValue({ status: 'draft' });
+
+    await invalidateApprovalIfNeeded({
+      tx: tx as never,
+      documentId: 'doc_1',
+    });
+
+    expect(tx.$executeRaw).toHaveBeenCalledTimes(1);
+    // Lock is taken before the status read (serializes vs approve()).
+    expect(tx.$executeRaw.mock.invocationCallOrder[0]).toBeLessThan(
+      tx.ismsDocument.findUnique.mock.invocationCallOrder[0],
+    );
+  });
+
+  it('reverts an approved document to draft (clearing approver + approvedAt)', async () => {
+    const tx = makeTx();
+    tx.ismsDocument.findUnique.mockResolvedValue({ status: 'approved' });
+
+    await invalidateApprovalIfNeeded({
+      tx: tx as never,
+      documentId: 'doc_1',
+    });
+
+    expect(tx.ismsDocument.update).toHaveBeenCalledWith({
+      where: { id: 'doc_1' },
+      data: { status: 'draft', approvedAt: null, approverId: null },
+    });
+  });
+
+  it('is a no-op for a non-approved document (but still locked)', async () => {
+    const tx = makeTx();
+    tx.ismsDocument.findUnique.mockResolvedValue({ status: 'needs_review' });
+
+    await invalidateApprovalIfNeeded({
+      tx: tx as never,
+      documentId: 'doc_1',
+    });
+
+    expect(tx.$executeRaw).toHaveBeenCalledTimes(1);
+    expect(tx.ismsDocument.update).not.toHaveBeenCalled();
+  });
+});
diff --git a/apps/api/src/isms/utils/approval.ts b/apps/api/src/isms/utils/approval.ts
index 186bfcae04..6b0db62544 100644
--- a/apps/api/src/isms/utils/approval.ts
+++ b/apps/api/src/isms/utils/approval.ts
@@ -1,4 +1,5 @@
 import type { Prisma } from '@db';
+import { lockDocument } from './document-lock';
 
 /**
  * Editing an approved ISMS document invalidates its sign-off: revert it to draft
@@ -6,6 +7,15 @@ import type { Prisma } from '@db';
  * touches the document when it is currently `approved`; a no-op otherwise. Must
  * run inside the same transaction as the content write so a failed write does not
  * leave the document reverted to draft without the new content.
+ *
+ * Every content-mutation path (register create/update/delete, narrative save,
+ * control link add/remove, regenerate) calls this, so acquiring the per-document
+ * advisory lock here centrally serializes ALL edits against approve() — which
+ * takes the same lock. Without it, an edit could interleave while approve() is
+ * loading + freezing the version under READ COMMITTED, leaving an approved version
+ * that omits the edit (or an edit that never invalidated the approval). The lock
+ * is transaction-scoped and re-entrant, so create paths that already take it for
+ * position allocation are unaffected.
  */
 export async function invalidateApprovalIfNeeded({
   tx,
@@ -14,6 +24,8 @@ export async function invalidateApprovalIfNeeded({
   tx: Prisma.TransactionClient;
   documentId: string;
 }): Promise<void> {
+  await lockDocument(tx, documentId);
+
   const document = await tx.ismsDocument.findUnique({
     where: { id: documentId },
     select: { status: true },