diff --git a/README.md b/README.md
index dd1ba7ac..2498c9ed 100644
--- a/README.md
+++ b/README.md
@@ -35,6 +35,8 @@ Session data is _not_ saved in the cookie itself, just the session ID.
   - `genid` - function to call to generate a new session ID. (default: uses `uid2` library)
   - `rolling` - forces a cookie set on every response. This resets the expiration date. (default: `false`)
   - `resave` - forces session to be saved even when unmodified. (default: `true`)
+  - `getSessionId` - a function that takes the incoming request as an argument and returns the session id.  If it returns null or undefined it still falls back to using the cookie.
+    - Example: `function(req) { return req.query.session_id }`.
   - `proxy` - trust the reverse proxy when setting secure cookies (via "x-forwarded-proto" header). When set to `true`, the "x-forwarded-proto" header will be used. When set to `false`, all headers are ignored. When left unset, will use the "trust proxy" setting from express. (default: `undefined`)
   - `saveUninitialized` - forces a session that is "uninitialized" to be saved to the store. A session is uninitialized when it is new but not modified. This is useful for implementing login sessions, reducing server storage usage, or complying with laws that require permission before setting a cookie. (default: `true`)
   - `unset` - controls result of unsetting `req.session` (through `delete`, setting to `null`, etc.). This can be "keep" to keep the session in the store but ignore modifications or "destroy" to destroy the stored session. (default: `'keep'`)
diff --git a/index.js b/index.js
index 35ea5a84..dbdc12e0 100644
--- a/index.js
+++ b/index.js
@@ -310,6 +310,8 @@ function session(options){
         ? saveUninitializedSession || isModified(req.session)
         : req.session.cookie.expires != null && isModified(req.session);
     }
+    // get the sessionID from the cookie
+    req.sessionID = ( options.getSessionID ? options.getSessionID(req) : null ) || unsignedCookie;
 
     // generate a session if the browser doesn't send a sessionID
     if (!req.sessionID) {