diff --git a/OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignal.java b/OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignal.java
index 24b23d92cd..15626b5d57 100644
--- a/OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignal.java
+++ b/OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignal.java
@@ -1414,6 +1414,41 @@ public void run() {
       emailLogout.run();
    }
 
+   public static void setExternalUserId(final String externalId) {
+
+      if (shouldLogUserPrivacyConsentErrorMessageForMethodName("setExternalId()"))
+         return;
+
+      Runnable runSetExternalUserId = new Runnable() {
+         @Override
+         public void run() {
+            try {
+               OneSignalStateSynchronizer.setExternalUserId(externalId);
+            } catch (JSONException exception) {
+               String operation = externalId == "" ? "remove" : "set";
+               onesignalLog(LOG_LEVEL.ERROR, "Attempted to " + operation + " external ID but encountered a JSON exception");
+               exception.printStackTrace();
+            }
+         }
+      };
+
+      // If either the app context is null or the waiting queue isn't done (to preserve operation order)
+      if (appContext == null || shouldRunTaskThroughQueue()) {
+         addTaskToQueue(new PendingTaskRunnable(runSetExternalUserId));
+         return;
+      }
+
+      runSetExternalUserId.run();
+   }
+
+   public static void removeExternalUserId() {
+      if (shouldLogUserPrivacyConsentErrorMessageForMethodName("removeExternalUserId()"))
+         return;
+
+      // to remove the external user ID, the API requires an empty string
+      setExternalUserId("");
+   }
+
    /**
     * Tag a user based on an app event of your choosing so later you can create
     * <a href="https://documentation.onesignal.com/docs/segmentation">OneSignal Segments</a>
diff --git a/OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignalStateSynchronizer.java b/OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignalStateSynchronizer.java
index b9742a24ef..e5ab2a64d1 100644
--- a/OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignalStateSynchronizer.java
+++ b/OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignalStateSynchronizer.java
@@ -165,4 +165,9 @@ static void logoutEmail() {
       getPushStateSynchronizer().logoutEmail();
       getEmailStateSynchronizer().logoutEmail();
    }
+
+   static void setExternalUserId(String externalId) throws JSONException {
+      getPushStateSynchronizer().setExternalUserId(externalId);
+      getEmailStateSynchronizer().setExternalUserId(externalId);
+   }
 }
\ No newline at end of file
diff --git a/OneSignalSDK/onesignal/src/main/java/com/onesignal/UserStateSynchronizer.java b/OneSignalSDK/onesignal/src/main/java/com/onesignal/UserStateSynchronizer.java
index 6728f96f50..8f23b5dcc3 100644
--- a/OneSignalSDK/onesignal/src/main/java/com/onesignal/UserStateSynchronizer.java
+++ b/OneSignalSDK/onesignal/src/main/java/com/onesignal/UserStateSynchronizer.java
@@ -454,6 +454,10 @@ void syncHashedEmail(JSONObject emailFields) {
         generateJsonDiff(syncValues, emailFields, syncValues, null);
     }
 
+    void setExternalUserId(final String externalId) throws JSONException {
+        getUserStateForModification().syncValues.put("external_user_id", externalId);
+    }
+
     abstract void setSubscription(boolean enable);
 
     private void handlePlayerDeletedFromServer() {
diff --git a/OneSignalSDK/unittest/src/test/java/com/test/onesignal/MainOneSignalClassRunner.java b/OneSignalSDK/unittest/src/test/java/com/test/onesignal/MainOneSignalClassRunner.java
index 130ceed7ff..2736c094ae 100644
--- a/OneSignalSDK/unittest/src/test/java/com/test/onesignal/MainOneSignalClassRunner.java
+++ b/OneSignalSDK/unittest/src/test/java/com/test/onesignal/MainOneSignalClassRunner.java
@@ -3236,6 +3236,108 @@ public void notificationReceived(OSNotification notification) {
       assertNull(ShadowFirebaseAnalytics.lastEventString);
    }
 
+   @Test
+   public void shouldSendExternalUserIdAfterRegistration() throws Exception {
+      OneSignalInit();
+      threadAndTaskWait();
+
+      String testExternalId = "test_ext_id";
+
+      OneSignal.setExternalUserId(testExternalId);
+
+      threadAndTaskWait();
+
+      assertEquals(3, ShadowOneSignalRestClient.networkCallCount);
+
+      ShadowOneSignalRestClient.Request externalIdRequest = ShadowOneSignalRestClient.requests.get(2);
+      assertEquals(ShadowOneSignalRestClient.REST_METHOD.PUT, externalIdRequest.method);
+      assertEquals(testExternalId, externalIdRequest.payload.getString("external_user_id"));
+   }
+
+   @Test
+   public void shouldSendExternalUserIdBeforeRegistration() throws Exception {
+      String testExternalId = "test_ext_id";
+
+      OneSignal.setExternalUserId(testExternalId);
+
+      OneSignalInit();
+      threadAndTaskWait();
+
+      assertEquals(2, ShadowOneSignalRestClient.networkCallCount);
+
+      ShadowOneSignalRestClient.Request registrationRequest = ShadowOneSignalRestClient.requests.get(1);
+      assertEquals(ShadowOneSignalRestClient.REST_METHOD.POST, registrationRequest.method);
+      assertEquals(testExternalId, registrationRequest.payload.getString("external_user_id"));
+   }
+
+   @Test
+   public void shouldRemoveExternalUserId() throws Exception {
+      OneSignal.setExternalUserId("test_ext_id");
+
+      OneSignalInit();
+      threadAndTaskWait();
+
+      OneSignal.removeExternalUserId();
+      threadAndTaskWait();
+
+      assertEquals(3, ShadowOneSignalRestClient.networkCallCount);
+
+      ShadowOneSignalRestClient.Request removeIdRequest = ShadowOneSignalRestClient.requests.get(2);
+      assertEquals(ShadowOneSignalRestClient.REST_METHOD.PUT, removeIdRequest.method);
+      assertEquals(removeIdRequest.payload.getString("external_user_id"), "");
+   }
+
+   @Test
+   public void doesNotSendSameExternalId() throws Exception {
+      String testExternalId = "test_ext_id";
+
+      OneSignal.setExternalUserId(testExternalId);
+
+      OneSignalInit();
+      threadAndTaskWait();
+
+      assertEquals(2, ShadowOneSignalRestClient.networkCallCount);
+
+      OneSignal.setExternalUserId(testExternalId);
+      threadAndTaskWait();
+
+      // Setting the same ID again should not generate a duplicate API request
+      // The SDK should detect it is the same and not generate a request
+      assertEquals(2, ShadowOneSignalRestClient.networkCallCount);
+   }
+
+   @Test
+   public void sendsExternalIdOnEmailPlayers() throws Exception {
+      String testExternalId = "test_ext_id";
+
+      OneSignalInit();
+      threadAndTaskWait();
+
+      OneSignal.setEmail("brad@onesignal.com");
+      threadAndTaskWait();
+
+      int currentRequestCount = ShadowOneSignalRestClient.networkCallCount;
+
+      OneSignal.setExternalUserId(testExternalId);
+      threadAndTaskWait();
+
+      // the SDK should have made two additional API calls
+      // One to set extID on the push player record,
+      // and another for the email player record
+      assertEquals(ShadowOneSignalRestClient.networkCallCount, currentRequestCount + 2);
+
+      int externalIdRequests = 0;
+
+      for (ShadowOneSignalRestClient.Request request : ShadowOneSignalRestClient.requests) {
+         if (request.payload != null && request.payload.has("external_user_id")) {
+            externalIdRequests += 1;
+            assertEquals(request.payload.getString("external_user_id"), testExternalId);
+         }
+      }
+
+      assertEquals(externalIdRequests, 2);
+   }
+
    // ####### Unit test helper methods ########
 
    private static OSNotification createTestOSNotification() throws Exception {