diff --git a/CHANGELOG.md b/CHANGELOG.md
index 438b8d0..4564096 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ Breaking changes:
 
 New features:
 - Add `prependArray` (#224 by @JordanMartinez)
+- Add `Data.Array.ST.length` (#239 by @Blugatroff)
 
 Bugfixes:
 
diff --git a/src/Data/Array/ST.js b/src/Data/Array/ST.js
index 57709b7..7dfff14 100644
--- a/src/Data/Array/ST.js
+++ b/src/Data/Array/ST.js
@@ -27,6 +27,12 @@ export const poke = function (i) {
   };
 };
 
+export const length = function (xs) {
+  return function () {
+    return xs.length;
+  };
+};
+
 export const popImpl = function (just) {
   return function (nothing) {
     return function (xs) {
diff --git a/src/Data/Array/ST.purs b/src/Data/Array/ST.purs
index 2817bee..b4e6a87 100644
--- a/src/Data/Array/ST.purs
+++ b/src/Data/Array/ST.purs
@@ -11,6 +11,7 @@ module Data.Array.ST
   , peek
   , poke
   , modify
+  , length
   , pop
   , push
   , pushAll
@@ -149,6 +150,9 @@ foreign import peekImpl
 -- | Change the value at the specified index in a mutable array.
 foreign import poke :: forall h a. Int -> a -> STArray h a -> ST h Boolean
 
+-- | Get the number of elements in a mutable array.
+foreign import length :: forall h a. STArray h a -> ST h Int
+
 -- | Remove the last element from an array and return that element.
 pop :: forall h a. STArray h a -> ST h (Maybe a)
 pop = popImpl Just Nothing
diff --git a/test/Test/Data/Array/ST.purs b/test/Test/Data/Array/ST.purs
index 8a83cd9..6e42fc2 100644
--- a/test/Test/Data/Array/ST.purs
+++ b/test/Test/Data/Array/ST.purs
@@ -49,6 +49,23 @@ testArrayST = do
 
   assert $ STA.run (STA.unsafeThaw [1, 2, 3]) == [1, 2, 3]
 
+  log "length should return the number of items in an STArray"
+
+  assert $ ST.run (do
+    arr <- STA.thaw nil
+    length <- STA.length arr
+    pure $ length == 0)
+
+  assert $ ST.run (do
+    arr <- STA.thaw [1]
+    length <- STA.length arr
+    pure $ length == 1)
+
+  assert $ ST.run (do
+    arr <- STA.thaw [1, 2, 3, 4, 5]
+    length <- STA.length arr
+    pure $ length == 5)
+
   log "pop should remove elements from an STArray"
 
   assert $ STA.run (do