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,
@@ -355,6 +356,12 @@
     }
   }
 
+  // 4. Complete login with call to `successfulLogInQueries(...)`
+  //    if `address` corresponds to an existing user.
+  const userID = await fetchUserIDForEthereumAddress(siweMessage.address);
+  if (userID) {
+    successfulLogInQueries(viewer, input, userID);
+  }
   return false;
 }