diff --git a/keyserver/src/creators/account-creator.js b/keyserver/src/creators/account-creator.js
--- a/keyserver/src/creators/account-creator.js
+++ b/keyserver/src/creators/account-creator.js
@@ -72,7 +72,10 @@
   if (request.password.trim() === '') {
     throw new ServerError('empty_password');
   }
-  if (request.username.search(validUsernameRegex) === -1) {
+  if (
+    request.username.search(validUsernameRegex) === -1 ||
+    isValidEthereumAddress(request.username.toLowerCase())
+  ) {
     throw new ServerError('invalid_username');
   }
 
@@ -87,10 +90,7 @@
   }
 
   const [existingUser, admin] = await Promise.all(promises);
-  if (
-    reservedUsernamesSet.has(request.username.toLowerCase()) ||
-    isValidEthereumAddress(request.username.toLowerCase())
-  ) {
+  if (reservedUsernamesSet.has(request.username.toLowerCase())) {
     throw new ServerError('username_reserved');
   }
   if (existingUser) {
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 { isValidEthereumAddress } from 'lib/utils/siwe-utils.js';
 
 import RegistrationButtonContainer from './registration-button-container.react.js';
 import RegistrationButton from './registration-button.react.js';
@@ -53,7 +54,9 @@
   const [username, setUsername] = React.useState(
     cachedSelections.username ?? '',
   );
-  const validUsername = username.search(validUsernameRegex) > -1;
+  const validUsername =
+    username.search(validUsernameRegex) > -1 &&
+    !isValidEthereumAddress(username.toLowerCase());
 
   const [usernameError, setUsernameError] = React.useState<?UsernameError>();
   const checkUsernameValidity = React.useCallback(() => {