diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,7 @@
+## 0.5.9
+
+* Add `Semigroup` instances for GHC 8.4 [#320](https://github.com/snoyberg/http-client/pull/320)
+
 ## 0.5.8
 
 * Switch to the new STM-based 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
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE DeriveFoldable #-}
 {-# LANGUAGE DeriveFunctor #-}
@@ -44,7 +45,8 @@
 import Blaze.ByteString.Builder (Builder, fromLazyByteString, fromByteString, toLazyByteString)
 import Data.Int (Int64)
 import Data.Foldable (Foldable)
-import Data.Monoid
+import Data.Monoid (Monoid(..))
+import Data.Semigroup (Semigroup(..))
 import Data.String (IsString, fromString)
 import Data.Time (UTCTime)
 import Data.Traversable (Traversable)
@@ -277,16 +279,21 @@
 instance Eq CookieJar where
   (==) cj1 cj2 = (DL.sort $ expose cj1) == (DL.sort $ expose cj2)
 
--- | Since 1.9
-instance Data.Monoid.Monoid CookieJar where
-  mempty = CJ []
-  (CJ a) `mappend` (CJ b) = CJ (DL.nub $ DL.sortBy compare' $ a `mappend` b)
+instance Semigroup CookieJar where
+  (CJ a) <> (CJ b) = CJ (DL.nub $ DL.sortBy compare' $ a <> b)
     where compare' c1 c2 =
             -- inverse so that recent cookies are kept by nub over older
             if cookie_creation_time c1 > cookie_creation_time c2
                 then LT
                 else GT
 
+-- | Since 1.9
+instance Data.Monoid.Monoid CookieJar where
+  mempty = CJ []
+#if !(MIN_VERSION_base(4,11,0))
+  mappend = (<>)
+#endif
+
 -- | Define a HTTP proxy, consisting of a hostname and port number.
 
 data Proxy = Proxy
@@ -323,9 +330,14 @@
     fromString str = RequestBodyBS (fromString str)
 instance Monoid RequestBody where
     mempty = RequestBodyBS S.empty
-    mappend x0 y0 =
+#if !(MIN_VERSION_base(4,11,0))
+    mappend = (<>)
+#endif
+
+instance Semigroup RequestBody where
+    x0 <> y0 =
         case (simplify x0, simplify y0) of
-            (Left (i, x), Left (j, y)) -> RequestBodyBuilder (i + j) (x `mappend` y)
+            (Left (i, x), Left (j, y)) -> RequestBodyBuilder (i + j) (x <> y)
             (Left x, Right y) -> combine (builderToStream x) y
             (Right x, Left y) -> combine x (builderToStream y)
             (Right x, Right y) -> combine x y
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:             0.5.8
+version:             0.5.9
 synopsis:            An HTTP client engine
 description:         Hackage documentation generation is not reliable. For up to date documentation, please see: <http://www.stackage.org/package/http-client>.
 homepage:            https://github.com/snoyberg/http-client
@@ -37,7 +37,7 @@
                        Network.PublicSuffixList.Serialize
                        Network.PublicSuffixList.DataStructure
                        Data.KeyedPool
-  build-depends:       base              >= 4.5    && < 5
+  build-depends:       base              >= 4.6    && < 5
                      , bytestring        >= 0.10
                      , text              >= 0.11
                      , http-types        >= 0.8
@@ -45,7 +45,7 @@
                      , time              >= 1.2
                      , network           >= 2.4
                      , streaming-commons >= 0.1.0.2 && < 0.3
-                     , containers
+                     , containers        >= 0.5
                      , transformers
                      , deepseq           >= 1.3    && <1.5
                      , case-insensitive  >= 1.0
@@ -57,11 +57,14 @@
                      , filepath
                      , mime-types
                      , ghc-prim
-                     , stm
+                     , stm               >= 2.3
   if flag(network-uri)
     build-depends: network >= 2.6, network-uri >= 2.6
   else
     build-depends: network < 2.6
+
+  if !impl(ghc>=8.0)
+    build-depends: semigroups >= 0.16.1
 
   if os(mingw32)
     build-depends: Win32, safe