diff --git a/.changeset/strict-identify-validation.md b/.changeset/strict-identify-validation.md
new file mode 100644
index 0000000000..162a571bfb
--- /dev/null
+++ b/.changeset/strict-identify-validation.md
@@ -0,0 +1,5 @@
+---
+'posthog-js': minor
+---
+
+Reject the strings "undefined" and "null" in posthog.identify(). All invalid distinct IDs now log a critical console error (always visible, not debug-only).
diff --git a/packages/browser/src/__tests__/identify.test.ts b/packages/browser/src/__tests__/identify.test.ts
index cccf809f99..406f4b0b79 100644
--- a/packages/browser/src/__tests__/identify.test.ts
+++ b/packages/browser/src/__tests__/identify.test.ts
@@ -41,6 +41,35 @@ describe('identify', () => {
         )
     })
 
+    describe('invalid distinct_id', () => {
+        it.each([
+            ['undefined', undefined, 'Unique user id has not been set in posthog.identify'],
+            ['null', null, 'Unique user id has not been set in posthog.identify'],
+            ['empty string', '', 'Unique user id has not been set in posthog.identify'],
+            ['whitespace only', '   ', 'Unique user id has not been set in posthog.identify'],
+            ['false', false, 'Unique user id has not been set in posthog.identify'],
+            [
+                'the string "undefined"',
+                'undefined',
+                'The string "undefined" was set in posthog.identify which indicates an error. This ID should be unique to the user and not a hardcoded string.',
+            ],
+            [
+                'the string "null"',
+                'null',
+                'The string "null" was set in posthog.identify which indicates an error. This ID should be unique to the user and not a hardcoded string.',
+            ],
+        ])('should reject %s and log a critical error', async (_label, invalidId, expectedMessage) => {
+            const token = uuidv7()
+            const beforeSendMock = jest.fn().mockImplementation((e) => e)
+            const posthog = await createPosthogInstance(token, { before_send: beforeSendMock })
+
+            posthog.identify(invalidId as any)
+
+            expect(beforeSendMock).not.toHaveBeenCalled()
+            expect(mockLogger.critical).toHaveBeenCalledWith(expectedMessage)
+        })
+    })
+
     it('should send $is_identified = true with the identify event and following events', async () => {
         // arrange
         const token = uuidv7()
diff --git a/packages/browser/src/posthog-core.ts b/packages/browser/src/posthog-core.ts
index f08fc72212..177fda4bb8 100644
--- a/packages/browser/src/posthog-core.ts
+++ b/packages/browser/src/posthog-core.ts
@@ -2290,6 +2290,26 @@ export class PostHog implements PostHogInterface {
         )
     }
 
+    private _validateIdentifyId(id: string | undefined): id is string {
+        if (!id || isEmptyString(id)) {
+            logger.critical('Unique user id has not been set in posthog.identify')
+            return false
+        }
+        if (id === COOKIELESS_SENTINEL_VALUE) {
+            logger.critical(
+                `The string "${id}" was set in posthog.identify which indicates an error. This ID is only used as a sentinel value.`
+            )
+            return false
+        }
+        if (isDistinctIdStringLike(id) || ['undefined', 'null'].includes(id.toLowerCase())) {
+            logger.critical(
+                `The string "${id}" was set in posthog.identify which indicates an error. This ID should be unique to the user and not a hardcoded string.`
+            )
+            return false
+        }
+        return true
+    }
+
     /**
      * Associates a user with a unique identifier instead of an auto-generated ID.
      * Learn more about [identifying users](/docs/product-analytics/identify)
@@ -2342,22 +2362,7 @@ export class PostHog implements PostHogInterface {
             )
         }
 
-        //if the new_distinct_id has not been set ignore the identify event
-        if (!new_distinct_id) {
-            logger.error('Unique user id has not been set in posthog.identify')
-            return
-        }
-
-        if (isDistinctIdStringLike(new_distinct_id)) {
-            logger.critical(
-                `The string "${new_distinct_id}" was set in posthog.identify which indicates an error. This ID should be unique to the user and not a hardcoded string.`
-            )
-            return
-        }
-        if (new_distinct_id === COOKIELESS_SENTINEL_VALUE) {
-            logger.critical(
-                `The string "${COOKIELESS_SENTINEL_VALUE}" was set in posthog.identify which indicates an error. This ID is only used as a sentinel value.`
-            )
+        if (!this._validateIdentifyId(new_distinct_id)) {
             return
         }
 
diff --git a/packages/browser/terser-mangled-names.json b/packages/browser/terser-mangled-names.json
index 2c1debe16e..d3fe571fc8 100644
--- a/packages/browser/terser-mangled-names.json
+++ b/packages/browser/terser-mangled-names.json
@@ -562,6 +562,7 @@
         "_urlTriggerStatus",
         "_urlTriggers",
         "_validateEmail",
+        "_validateIdentifyId",
         "_validateSampleRate",
         "_visibilityChangeListener",
         "_visibilityStateListener",