diff --git a/keyserver/src/endpoints.js b/keyserver/src/endpoints.js
--- a/keyserver/src/endpoints.js
+++ b/keyserver/src/endpoints.js
@@ -178,6 +178,8 @@
   updatePasswordRequestInputValidator,
   updateUserAvatarResponderValidator,
   updateUserSettingsInputValidator,
+  claimUsernameResponder,
+  claimUsernameResponseValidator,
 } from './responders/user-responders.js';
 import {
   codeVerificationResponder,
@@ -533,6 +535,12 @@
     logInResponseValidator,
     [],
   ),
+  claim_username: createJSONResponder(
+    claimUsernameResponder,
+    ignoredArgumentValidator,
+    claimUsernameResponseValidator,
+    [],
+  ),
   update_user_avatar: createJSONResponder(
     updateUserAvatarResponder,
     updateUserAvatarRequestValidator,
diff --git a/keyserver/src/responders/user-responders.js b/keyserver/src/responders/user-responders.js
--- a/keyserver/src/responders/user-responders.js
+++ b/keyserver/src/responders/user-responders.js
@@ -23,6 +23,7 @@
   UpdatePasswordRequest,
   UpdateUserSettingsRequest,
   PolicyAcknowledgmentRequest,
+  ClaimUsernameResponse,
 } from 'lib/types/account-types.js';
 import {
   userSettingsTypes,
@@ -36,6 +37,7 @@
   type UpdateUserAvatarRequest,
 } from 'lib/types/avatar-types.js';
 import type {
+  ReservedUsernameMessage,
   IdentityKeysBlob,
   SignedIdentityKeysBlob,
 } from 'lib/types/crypto-types.js';
@@ -113,6 +115,7 @@
   fetchKnownUserInfos,
   fetchLoggedInUserInfo,
   fetchUserIDForEthereumAddress,
+  fetchUsername,
 } from '../fetchers/user-fetchers.js';
 import {
   createNewAnonymousCookie,
@@ -129,6 +132,7 @@
   updateUserSettings,
   updateUserAvatar,
 } from '../updaters/account-updaters.js';
+import { fetchOlmAccount } from '../updaters/olm-account-updater.js';
 import { userSubscriptionUpdater } from '../updaters/user-subscription-updaters.js';
 import { viewerAcknowledgmentUpdater } from '../updaters/viewer-acknowledgment-updater.js';
 import { getOlmUtility } from '../utils/olm-utils.js';
@@ -730,6 +734,36 @@
   return await updateUserAvatar(viewer, request);
 }
 
+export const claimUsernameResponseValidator: TInterface<ClaimUsernameResponse> =
+  tShape<ClaimUsernameResponse>({
+    message: t.String,
+    signature: t.String,
+  });
+
+async function claimUsernameResponder(
+  viewer: Viewer,
+): Promise<ClaimUsernameResponse> {
+  const [username, accountInfo] = await Promise.all([
+    fetchUsername(viewer.userID),
+    fetchOlmAccount('content'),
+  ]);
+
+  if (!username) {
+    throw new ServerError('invalid_credentials');
+  }
+
+  const issuedAt = new Date().toISOString();
+  const reservedUsernameMessage: ReservedUsernameMessage = {
+    statement: 'This user is the owner of the following username',
+    payload: username,
+    issuedAt,
+  };
+  const message = JSON.stringify(reservedUsernameMessage);
+  const signature = accountInfo.account.sign(message);
+
+  return { message, signature };
+}
+
 export {
   userSubscriptionUpdateResponder,
   passwordUpdateResponder,
@@ -744,4 +778,5 @@
   updateUserSettingsResponder,
   policyAcknowledgmentResponder,
   updateUserAvatarResponder,
+  claimUsernameResponder,
 };
diff --git a/lib/actions/user-actions.js b/lib/actions/user-actions.js
--- a/lib/actions/user-actions.js
+++ b/lib/actions/user-actions.js
@@ -9,6 +9,7 @@
   RegisterInfo,
   UpdateUserSettingsRequest,
   PolicyAcknowledgmentRequest,
+  ClaimUsernameResponse,
 } from '../types/account-types.js';
 import type {
   UpdateUserAvatarRequest,
@@ -62,6 +63,25 @@
     return { currentUserInfo, preRequestUserState };
   };
 
+const claimUsernameActionTypes = Object.freeze({
+  started: 'CLAIM_USERNAME_STARTED',
+  success: 'CLAIM_USERNAME_SUCCESS',
+  failed: 'CLAIM_USERNAME_FAILED',
+});
+const claimUsernameCallServerEndpointOptions = { timeout: 500 };
+const claimUsername =
+  (
+    callServerEndpoint: CallServerEndpoint,
+  ): (() => Promise<ClaimUsernameResponse>) =>
+  async () => {
+    const response = await callServerEndpoint(
+      'claim_username',
+      {},
+      { ...claimUsernameCallServerEndpointOptions },
+    );
+    return response;
+  };
+
 const deleteAccountActionTypes = Object.freeze({
   started: 'DELETE_ACCOUNT_STARTED',
   success: 'DELETE_ACCOUNT_SUCCESS',
@@ -327,6 +347,8 @@
 export {
   changeUserPasswordActionTypes,
   changeUserPassword,
+  claimUsernameActionTypes,
+  claimUsername,
   deleteAccount,
   deleteAccountActionTypes,
   getSessionPublicKeys,
diff --git a/lib/types/account-types.js b/lib/types/account-types.js
--- a/lib/types/account-types.js
+++ b/lib/types/account-types.js
@@ -203,3 +203,8 @@
   tShape<DefaultNotificationPayload>({
     default_user_notifications: t.maybe(t.enums.of(notificationTypeValues)),
   });
+
+export type ClaimUsernameResponse = {
+  +message: string,
+  +signature: string,
+};
diff --git a/lib/types/crypto-types.js b/lib/types/crypto-types.js
--- a/lib/types/crypto-types.js
+++ b/lib/types/crypto-types.js
@@ -59,6 +59,11 @@
       +statement: 'Remove the following username from reserved list',
       +payload: string,
       +issuedAt: string,
+    }
+  | {
+      +statement: 'This user is the owner of the following username',
+      +payload: string,
+      +issuedAt: string,
     };
 
 export const olmEncryptedMessageTypes = Object.freeze({
diff --git a/lib/types/endpoints.js b/lib/types/endpoints.js
--- a/lib/types/endpoints.js
+++ b/lib/types/endpoints.js
@@ -93,6 +93,7 @@
   VERIFY_INVITE_LINK: 'verify_invite_link',
   SIWE_NONCE: 'siwe_nonce',
   SIWE_AUTH: 'siwe_auth',
+  CLAIM_USERNAME: 'claim_username',
   UPDATE_USER_AVATAR: 'update_user_avatar',
   UPLOAD_MEDIA_METADATA: 'upload_media_metadata',
   SEARCH_MESSAGES: 'search_messages',
diff --git a/lib/types/redux-types.js b/lib/types/redux-types.js
--- a/lib/types/redux-types.js
+++ b/lib/types/redux-types.js
@@ -6,6 +6,7 @@
   LogInResult,
   RegisterResult,
   DefaultNotificationPayload,
+  ClaimUsernameResponse,
 } from './account-types.js';
 import type {
   ActivityUpdateSuccessPayload,
@@ -184,6 +185,22 @@
       +payload: LogOutResult,
       +loadingInfo: LoadingInfo,
     }
+  | {
+      +type: 'CLAIM_USERNAME_STARTED',
+      +payload?: void,
+      +loadingInfo: LoadingInfo,
+    }
+  | {
+      +type: 'CLAIM_USERNAME_FAILED',
+      +error: true,
+      +payload: Error,
+      +loadingInfo: LoadingInfo,
+    }
+  | {
+      +type: 'CLAIM_USERNAME_SUCCESS',
+      +payload: ClaimUsernameResponse,
+      +loadingInfo: LoadingInfo,
+    }
   | {
       +type: 'DELETE_ACCOUNT_STARTED',
       +payload?: void,