diff --git a/src/Data/Map.purs b/src/Data/Map.purs
index e764370b..798ef32a 100644
--- a/src/Data/Map.purs
+++ b/src/Data/Map.purs
@@ -32,6 +32,7 @@ module Data.Map
   , union
   , unionWith
   , unions
+  , unionsWith
   , isSubmap
   , size
   , mapWithKey
@@ -608,6 +609,9 @@ union = unionWith const
 unions :: forall k v f. Ord k => Foldable f => f (Map k v) -> Map k v
 unions = foldl union empty
 
+unionsWith :: forall k v f. Ord k => Foldable f => (v -> v -> v) -> f (Map k v) -> Map k v
+unionsWith f = foldl (unionWith f) empty
+
 -- | Test whether one map contains all of the keys and values contained in another map
 isSubmap :: forall k v. Ord k => Eq v => Map k v -> Map k v -> Boolean
 isSubmap m1 m2 = LL.all f $ (toUnfoldable m1 :: LL.List (Tuple k v))
diff --git a/test/Test/Data/Map.purs b/test/Test/Data/Map.purs
index bc38e615..e53f81aa 100644
--- a/test/Test/Data/Map.purs
+++ b/test/Test/Data/Map.purs
@@ -23,6 +23,9 @@ import Test.QuickCheck.Gen (elements, oneOf)
 
 newtype TestMap k v = TestMap (M.Map k v)
 
+unwrap :: forall k v. TestMap k v -> M.Map k v
+unwrap (TestMap m) = m
+
 instance arbTestMap :: (Eq k, Ord k, Arbitrary k, Arbitrary v) => Arbitrary (TestMap k v) where
   arbitrary = TestMap <$> genMap arbitrary arbitrary
 
@@ -214,6 +217,26 @@ mapTests = do
           Just v              -> Just v == v2
           Nothing             -> not (in1 || in2)
 
+  log "unionsWith"
+  for_ [Tuple (+) 0, Tuple (*) 1] $ \(Tuple op ident) ->
+    quickCheck $ \(testMaps :: Array (TestMap SmallKey Int)) k ->
+      let testMaps' = unwrap <$> testMaps
+          u = M.unionsWith op testMaps'
+      in case M.lookup k u of
+           Nothing -> A.all (not <<< M.member k) testMaps'
+           Just v -> (v == _) <<< A.foldl op ident <<< map (fromMaybe ident <<< M.lookup k) $ testMaps'
+
+  log "unionsWith argument order"
+  quickCheck $ \(testMaps :: Array (TestMap SmallKey Int)) k ->
+    let testMaps' = unwrap <$> testMaps
+        u = M.unionsWith (-) testMaps'
+    in case M.lookup k u of
+         Just v ->
+           case A.uncons <<< A.mapMaybe (M.lookup k) $ testMaps' of
+             Nothing -> false
+             Just { head, tail } -> v == foldl (-) head tail
+         Nothing -> A.all (not <<< M.member k) testMaps'
+
   log "size"
   quickCheck $ \xs ->
     let xs' = nubBy ((==) `on` fst) xs