diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,7 @@
+## 0.4.4
+
+Add `managerModifyRequest` field to `ManagerSettings`.
+
 ## 0.4.3
 
 Add `requestVersion` field to `Request`.
diff --git a/Network/HTTP/Client.hs b/Network/HTTP/Client.hs
--- a/Network/HTTP/Client.hs
+++ b/Network/HTTP/Client.hs
@@ -103,6 +103,7 @@
     , managerRetryableException
     , managerWrapIOException
     , managerIdleConnectionCount
+    , managerModifyRequest
       -- *** Helpers
     , rawConnectionModifySocket
       -- * Request
diff --git a/Network/HTTP/Client/Core.hs b/Network/HTTP/Client/Core.hs
--- a/Network/HTTP/Client/Core.hs
+++ b/Network/HTTP/Client/Core.hs
@@ -71,7 +71,8 @@
      :: Request
      -> Manager
      -> IO (Response BodyReader)
-httpRaw req' m = do
+httpRaw req0 m = do
+    req' <- mModifyRequest m req0
     (req, cookie_jar') <- case cookieJar req' of
         Just cj -> do
             now <- getCurrentTime
diff --git a/Network/HTTP/Client/Manager.hs b/Network/HTTP/Client/Manager.hs
--- a/Network/HTTP/Client/Manager.hs
+++ b/Network/HTTP/Client/Manager.hs
@@ -92,6 +92,7 @@
                     Nothing -> se
          in handle $ throwIO . wrapper
     , managerIdleConnectionCount = 512
+    , managerModifyRequest = return
     }
 
 takeSocket :: Manager -> ConnKey -> IO (Maybe Connection)
@@ -169,6 +170,7 @@
             , mRetryableException = managerRetryableException ms
             , mWrapIOException = managerWrapIOException ms
             , mIdleConnectionCount = managerIdleConnectionCount ms
+            , mModifyRequest = managerModifyRequest ms
             }
     return manager
 
diff --git a/Network/HTTP/Client/Types.hs b/Network/HTTP/Client/Types.hs
--- a/Network/HTTP/Client/Types.hs
+++ b/Network/HTTP/Client/Types.hs
@@ -502,6 +502,12 @@
     -- Default: 512
     --
     -- Since 0.3.7
+    , managerModifyRequest :: Request -> IO Request
+    -- ^ Perform the given modification to a @Request@ before performing it.
+    --
+    -- Default: no modification
+    --
+    -- Since 0.4.4
     }
     deriving T.Typeable
 
@@ -529,6 +535,7 @@
     , mRetryableException :: SomeException -> Bool
     , mWrapIOException :: forall a. IO a -> IO a
     , mIdleConnectionCount :: Int
+    , mModifyRequest :: Request -> IO Request
     }
     deriving T.Typeable
 
diff --git a/http-client.cabal b/http-client.cabal
--- a/http-client.cabal
+++ b/http-client.cabal
@@ -1,5 +1,5 @@
 name:                http-client
-version:             0.4.3
+version:             0.4.4
 synopsis:            An HTTP client engine, intended as a base layer for more user-friendly packages.
 description:         This codebase has been refactored from http-conduit.
 homepage:            https://github.com/snoyberg/http-client
diff --git a/test/Network/HTTP/ClientSpec.hs b/test/Network/HTTP/ClientSpec.hs
--- a/test/Network/HTTP/ClientSpec.hs
+++ b/test/Network/HTTP/ClientSpec.hs
@@ -104,3 +104,10 @@
                     responseBody x `shouldBe` "hello"
         test False
         test True
+
+    it "managerModifyRequest" $ do
+        let modify req = return req { port = 80 }
+            settings = defaultManagerSettings { managerModifyRequest = modify }
+        withManager settings $ \man -> do
+            res <- httpLbs "http://httpbin.org:1234" man
+            responseStatus res `shouldBe` status200