diff --git a/native/account/registration/username-selection.react.js b/native/account/registration/username-selection.react.js
--- a/native/account/registration/username-selection.react.js
+++ b/native/account/registration/username-selection.react.js
@@ -12,6 +12,7 @@
 import { validUsernameRegex } from 'lib/shared/account-utils.js';
 import { useLegacyAshoatKeyserverCall } from 'lib/utils/action-utils.js';
 import { useDispatchActionPromise } from 'lib/utils/redux-promise-utils.js';
+import { usingCommServicesAccessToken } from 'lib/utils/services-utils.js';
 import { isValidEthereumAddress } from 'lib/utils/siwe-utils.js';
 
 import RegistrationButtonContainer from './registration-button-container.react.js';
@@ -22,6 +23,7 @@
 import type { RegistrationNavigationProp } from './registration-navigator.react.js';
 import RegistrationTextInput from './registration-text-input.react.js';
 import type { CoolOrNerdMode } from './registration-types.js';
+import { commRustModule } from '../../native-modules.js';
 import {
   type NavigationRoute,
   PasswordSelectionRouteName,
@@ -88,11 +90,21 @@
       return;
     }
 
-    const searchPromise = exactSearchUserCall(username);
-    void dispatchActionPromise(exactSearchUserActionTypes, searchPromise);
-    const { userInfo } = await searchPromise;
+    let userAlreadyExists;
+    if (usingCommServicesAccessToken) {
+      const findUserIDResponseString =
+        await commRustModule.findUserIDForUsername(username);
+      const findUserIDResponse = JSON.parse(findUserIDResponseString);
+      userAlreadyExists =
+        !!findUserIDResponse.userID || findUserIDResponse.isReserved;
+    } else {
+      const searchPromise = exactSearchUserCall(username);
+      void dispatchActionPromise(exactSearchUserActionTypes, searchPromise);
+      const { userInfo } = await searchPromise;
+      userAlreadyExists = !!userInfo;
+    }
 
-    if (userInfo) {
+    if (userAlreadyExists) {
       setUsernameError('username_taken');
       return;
     }
diff --git a/native/cpp/CommonCpp/NativeModules/CommRustModule.h b/native/cpp/CommonCpp/NativeModules/CommRustModule.h
--- a/native/cpp/CommonCpp/NativeModules/CommRustModule.h
+++ b/native/cpp/CommonCpp/NativeModules/CommRustModule.h
@@ -125,6 +125,8 @@
   virtual jsi::Value findUserIDForWalletAddress(
       jsi::Runtime &rt,
       jsi::String walletAddress) override;
+  virtual jsi::Value
+  findUserIDForUsername(jsi::Runtime &rt, jsi::String username) override;
 
 public:
   CommRustModule(std::shared_ptr<facebook::react::CallInvoker> jsInvoker);
diff --git a/native/cpp/CommonCpp/NativeModules/CommRustModule.cpp b/native/cpp/CommonCpp/NativeModules/CommRustModule.cpp
--- a/native/cpp/CommonCpp/NativeModules/CommRustModule.cpp
+++ b/native/cpp/CommonCpp/NativeModules/CommRustModule.cpp
@@ -600,4 +600,24 @@
       });
 }
 
+jsi::Value
+CommRustModule::findUserIDForUsername(jsi::Runtime &rt, jsi::String username) {
+  auto usernameRust = jsiStringToRustString(username, rt);
+  return createPromiseAsJSIValue(
+      rt, [=, this](jsi::Runtime &innerRt, std::shared_ptr<Promise> promise) {
+        std::string error;
+        try {
+          auto currentID = RustPromiseManager::instance.addPromise(
+              {promise, this->jsInvoker_, innerRt});
+          identityFindUserIDForUsername(usernameRust, currentID);
+        } catch (const std::exception &e) {
+          error = e.what();
+        };
+        if (!error.empty()) {
+          this->jsInvoker_->invokeAsync(
+              [error, promise]() { promise->reject(error); });
+        }
+      });
+}
+
 } // namespace comm
diff --git a/native/cpp/CommonCpp/_generated/rustJSI-generated.cpp b/native/cpp/CommonCpp/_generated/rustJSI-generated.cpp
--- a/native/cpp/CommonCpp/_generated/rustJSI-generated.cpp
+++ b/native/cpp/CommonCpp/_generated/rustJSI-generated.cpp
@@ -60,6 +60,9 @@
 static jsi::Value __hostFunction_CommRustModuleSchemaCxxSpecJSI_findUserIDForWalletAddress(jsi::Runtime &rt, TurboModule &turboModule, const jsi::Value* args, size_t count) {
   return static_cast<CommRustModuleSchemaCxxSpecJSI *>(&turboModule)->findUserIDForWalletAddress(rt, args[0].asString(rt));
 }
+static jsi::Value __hostFunction_CommRustModuleSchemaCxxSpecJSI_findUserIDForUsername(jsi::Runtime &rt, TurboModule &turboModule, const jsi::Value* args, size_t count) {
+  return static_cast<CommRustModuleSchemaCxxSpecJSI *>(&turboModule)->findUserIDForUsername(rt, args[0].asString(rt));
+}
 
 CommRustModuleSchemaCxxSpecJSI::CommRustModuleSchemaCxxSpecJSI(std::shared_ptr<CallInvoker> jsInvoker)
   : TurboModule("CommRustTurboModule", jsInvoker) {
@@ -79,6 +82,7 @@
   methodMap_["updateDeviceList"] = MethodMetadata {4, __hostFunction_CommRustModuleSchemaCxxSpecJSI_updateDeviceList};
   methodMap_["uploadSecondaryDeviceKeysAndLogIn"] = MethodMetadata {10, __hostFunction_CommRustModuleSchemaCxxSpecJSI_uploadSecondaryDeviceKeysAndLogIn};
   methodMap_["findUserIDForWalletAddress"] = MethodMetadata {1, __hostFunction_CommRustModuleSchemaCxxSpecJSI_findUserIDForWalletAddress};
+  methodMap_["findUserIDForUsername"] = MethodMetadata {1, __hostFunction_CommRustModuleSchemaCxxSpecJSI_findUserIDForUsername};
 }
 
 
diff --git a/native/cpp/CommonCpp/_generated/rustJSI.h b/native/cpp/CommonCpp/_generated/rustJSI.h
--- a/native/cpp/CommonCpp/_generated/rustJSI.h
+++ b/native/cpp/CommonCpp/_generated/rustJSI.h
@@ -36,6 +36,7 @@
   virtual jsi::Value updateDeviceList(jsi::Runtime &rt, jsi::String authUserID, jsi::String authDeviceID, jsi::String authAccessToken, jsi::String updatePayload) = 0;
   virtual jsi::Value uploadSecondaryDeviceKeysAndLogIn(jsi::Runtime &rt, jsi::String userID, jsi::String challengeResponse, jsi::String keyPayload, jsi::String keyPayloadSignature, jsi::String contentPrekey, jsi::String contentPrekeySignature, jsi::String notifPrekey, jsi::String notifPrekeySignature, jsi::Array contentOneTimeKeys, jsi::Array notifOneTimeKeys) = 0;
   virtual jsi::Value findUserIDForWalletAddress(jsi::Runtime &rt, jsi::String walletAddress) = 0;
+  virtual jsi::Value findUserIDForUsername(jsi::Runtime &rt, jsi::String username) = 0;
 
 };
 
@@ -185,6 +186,14 @@
       return bridging::callFromJs<jsi::Value>(
           rt, &T::findUserIDForWalletAddress, jsInvoker_, instance_, std::move(walletAddress));
     }
+    jsi::Value findUserIDForUsername(jsi::Runtime &rt, jsi::String username) override {
+      static_assert(
+          bridging::getParameterCount(&T::findUserIDForUsername) == 2,
+          "Expected findUserIDForUsername(...) to have 2 parameters");
+
+      return bridging::callFromJs<jsi::Value>(
+          rt, &T::findUserIDForUsername, jsInvoker_, instance_, std::move(username));
+    }
 
   private:
     T *instance_;
diff --git a/native/native_rust_library/src/exact_user_search.rs b/native/native_rust_library/src/exact_user_search.rs
--- a/native/native_rust_library/src/exact_user_search.rs
+++ b/native/native_rust_library/src/exact_user_search.rs
@@ -2,6 +2,7 @@
   handle_string_result_as_callback, Error, CODE_VERSION, DEVICE_TYPE,
   IDENTITY_SOCKET_ADDR, RUNTIME,
 };
+use find_user_id_request::Identifier as RequestIdentifier;
 use grpc_clients::identity::{
   get_unauthenticated_client,
   protos::unauth::{find_user_id_request, FindUserIdRequest},
@@ -14,6 +15,7 @@
 struct FindUserIDResponse {
   #[serde(rename = "userID")]
   user_id: Option<String>,
+  #[serde(rename = "isReserved")]
   is_reserved: bool,
 }
 
@@ -23,15 +25,27 @@
   promise_id: u32,
 ) {
   RUNTIME.spawn(async move {
-    let result = find_user_id_helper(wallet_address).await;
+    let result =
+      find_user_id_helper(RequestIdentifier::WalletAddress(wallet_address))
+        .await;
     handle_string_result_as_callback(result, promise_id);
   });
 }
 
-async fn find_user_id_helper(wallet_address: String) -> Result<String, Error> {
-  use find_user_id_request::Identifier as RequestIdentifier;
+#[instrument]
+pub fn find_user_id_for_username(username: String, promise_id: u32) {
+  RUNTIME.spawn(async move {
+    let result =
+      find_user_id_helper(RequestIdentifier::Username(username)).await;
+    handle_string_result_as_callback(result, promise_id);
+  });
+}
+
+async fn find_user_id_helper(
+  identifier: RequestIdentifier,
+) -> Result<String, Error> {
   let find_user_id_request = FindUserIdRequest {
-    identifier: Some(RequestIdentifier::WalletAddress(wallet_address)),
+    identifier: Some(identifier),
   };
 
   let mut identity_client = get_unauthenticated_client(
diff --git a/native/native_rust_library/src/lib.rs b/native/native_rust_library/src/lib.rs
--- a/native/native_rust_library/src/lib.rs
+++ b/native/native_rust_library/src/lib.rs
@@ -1,7 +1,9 @@
 use backup::ffi::*;
 use comm_opaque2::client::{Login, Registration};
 use comm_opaque2::grpc::opaque_error_to_grpc_status as handle_error;
-use exact_user_search::find_user_id_for_wallet_address;
+use exact_user_search::{
+  find_user_id_for_username, find_user_id_for_wallet_address,
+};
 use ffi::{bool_callback, string_callback, void_callback};
 use future_manager::ffi::*;
 use grpc_clients::identity::protos::auth::{
@@ -230,6 +232,9 @@
     #[cxx_name = "identityFindUserIDForWalletAddress"]
     fn find_user_id_for_wallet_address(wallet_address: String, promise_id: u32);
 
+    #[cxx_name = "identityFindUserIDForUsername"]
+    fn find_user_id_for_username(username: String, promise_id: u32);
+
     // Argon2
     #[cxx_name = "compute_backup_key"]
     fn compute_backup_key_str(
diff --git a/native/schema/CommRustModuleSchema.js b/native/schema/CommRustModuleSchema.js
--- a/native/schema/CommRustModuleSchema.js
+++ b/native/schema/CommRustModuleSchema.js
@@ -118,6 +118,7 @@
     notifOneTimeKeys: $ReadOnlyArray<string>,
   ) => Promise<string>;
   +findUserIDForWalletAddress: (walletAddress: string) => Promise<string>;
+  +findUserIDForUsername: (username: string) => Promise<string>;
 }
 
 export default (TurboModuleRegistry.getEnforcing<Spec>(