diff --git a/keyserver/src/fetchers/user-fetchers.js b/keyserver/src/fetchers/user-fetchers.js
--- a/keyserver/src/fetchers/user-fetchers.js
+++ b/keyserver/src/fetchers/user-fetchers.js
@@ -290,6 +290,18 @@
   return result[0].user;
 }
 
+async function fetchUserIDForEthereumAddress(
+  address: string,
+): Promise<?string> {
+  const query = SQL`
+    SELECT id
+    FROM users
+    WHERE LCASE(ethereum_address) = LCASE(${address})
+  `;
+  const [result] = await dbQuery(query);
+  return result.length === 0 ? null : result[0].id;
+}
+
 export {
   fetchUserInfos,
   fetchLoggedInUserInfo,
@@ -300,4 +312,5 @@
   fetchUsername,
   fetchKnownUserInfos,
   fetchKeyserverAdminID,
+  fetchUserIDForEthereumAddress,
 };
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
@@ -55,6 +55,7 @@
 import {
   fetchKnownUserInfos,
   fetchLoggedInUserInfo,
+  fetchUserIDForEthereumAddress,
 } from '../fetchers/user-fetchers';
 import {
   createNewAnonymousCookie,
@@ -195,7 +196,7 @@
   input: any,
   userID: string,
   calendarQuery: ?CalendarQuery,
-) {
+): Promise<LogInResponse> {
   const request: LogInRequest = input;
   const newServerTime = Date.now();
   const deviceToken = request.deviceTokenUpdateRequest
@@ -316,7 +317,10 @@
   watchedIDs: t.list(t.String),
 });
 
-async function siweAuthResponder(viewer: Viewer, input: any): Promise<boolean> {
+async function siweAuthResponder(
+  viewer: Viewer,
+  input: any,
+): Promise<LogInResponse> {
   await validateInput(viewer, siweAuthRequestInputValidator, input);
   const request: SIWEAuthRequest = input;
   const { message, signature } = request;
@@ -355,7 +359,13 @@
     }
   }
 
-  return false;
+  // 4. Complete login with call to `successfulLogInQueries(...)`
+  //    if `address` corresponds to an existing user.
+  const userID = await fetchUserIDForEthereumAddress(siweMessage.address);
+  if (!userID) {
+    throw new ServerError('placeholder_error');
+  }
+  return await processSuccessfulLogin(viewer, input, userID);
 }
 
 const updatePasswordRequestInputValidator = tShape({
diff --git a/lib/actions/siwe-actions.js b/lib/actions/siwe-actions.js
--- a/lib/actions/siwe-actions.js
+++ b/lib/actions/siwe-actions.js
@@ -1,6 +1,7 @@
 // @flow
 
 import threadWatcher from '../shared/thread-watcher.js';
+import type { LogInResponse } from '../types/account-types.js';
 import type { SIWEAuthServerCall } from '../types/siwe-types.js';
 import type { CallServerEndpoint } from '../utils/call-server-endpoint';
 import { getConfig } from '../utils/config.js';
@@ -27,7 +28,7 @@
   callServerEndpoint: CallServerEndpoint,
 ): ((
   siweAuthPayload: SIWEAuthServerCall,
-) => Promise<boolean>) => async siweAuthPayload => {
+) => Promise<LogInResponse>) => async siweAuthPayload => {
   const watchedIDs = threadWatcher.getWatchedIDs();
   const response = await callServerEndpoint(
     'siwe_auth',
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
@@ -7,6 +7,7 @@
   LogInResult,
   RegisterResult,
   DefaultNotificationPayload,
+  LogInResponse,
 } from './account-types';
 import type {
   ActivityUpdateSuccessPayload,
@@ -901,7 +902,7 @@
     }
   | {
       +type: 'SIWE_AUTH_SUCCESS',
-      +payload?: boolean,
+      +payload: LogInResponse,
       +loadingInfo: LoadingInfo,
     }
   | {