diff --git a/Lib/heapq.py b/Lib/heapq.py
--- a/Lib/heapq.py
+++ b/Lib/heapq.py
@@ -311,7 +311,7 @@
 except ImportError:
     pass
 
-def merge(*iterables):
+def merge(*iterables, key=None):
     '''Merge multiple sorted inputs into a single sorted output.
 
     Similar to sorted(itertools.chain(*iterables)) but returns a generator,
@@ -321,31 +321,63 @@
     >>> list(merge([1,3,5,7], [0,2,4,8], [5,10,15,20], [], [25]))
     [0, 1, 2, 3, 4, 5, 5, 7, 8, 10, 15, 20, 25]
 
+    If *key* is not None, applies a key function to each element to determine
+    its sort order.
+
+    >>> list(merge(['dog', 'horse'], ['cat', 'fish', 'kangaroo'], key=len))
+    ['dog', 'cat', 'fish', 'horse', 'kangaroo']
+
     '''
 
     h = []
     h_append = h.append
+    _heapreplace = heapreplace
+
+    if key is None:
+        for order, it in enumerate(map(iter, iterables)):
+            try:
+                next = it.__next__
+                h_append([next(), order, next])
+            except StopIteration:
+                pass
+        heapify(h)
+        while len(h) > 1:
+            try:
+                while True:
+                    value, order, next = s = h[0]
+                    yield value
+                    s[0] = next()           # raises StopIteration when exhausted
+                    _heapreplace(h, s)      # restore heap condition
+            except StopIteration:
+                heappop(h)                  # remove empty iterator
+        if h:
+            # fast case when only a single iterator remains
+            value, order, next = h[0]
+            yield value
+            yield from next.__self__
+        return
+
     for order, it in enumerate(map(iter, iterables)):
         try:
             next = it.__next__
-            h_append([next(), order, next])
+            value = next()
+            h_append([key(value), order, value, next])
         except StopIteration:
             pass
     heapify(h)
-
-    _heapreplace = heapreplace
     while len(h) > 1:
         try:
             while True:
-                value, order, next = s = h[0]
+                key_value, order, value, next = s = h[0]
                 yield value
-                s[0] = next()           # raises StopIteration when exhausted
-                _heapreplace(h, s)      # restore heap condition
+                value = next()
+                s[0] = key(value)
+                s[2] = value
+                _heapreplace(h, s)
         except StopIteration:
-            heappop(h)                  # remove empty iterator
+            heappop(h)
     if h:
-        # fast case when only a single iterator remains
-        value, order, next = h[0]
+        key_value, order, value,  next = h[0]
         yield value
         yield from next.__self__
 
diff --git a/Lib/test/test_heapq.py b/Lib/test/test_heapq.py
--- a/Lib/test/test_heapq.py
+++ b/Lib/test/test_heapq.py
@@ -158,6 +158,17 @@
         self.assertEqual(sorted(chain(*inputs)), list(self.module.merge(*inputs)))
         self.assertEqual(list(self.module.merge()), [])
 
+    def test_merge_with_key_function(self):
+        inputs = []
+        for i in range(random.randrange(5)):
+            row = list(random.randrange(-1000, 1000)
+                       for j in range(random.randrange(10)))
+            row.sort(key=abs)
+            inputs.append(row)
+        self.assertEqual(sorted(chain(*inputs), key=abs),
+                         list(self.module.merge(*inputs, key=abs)))
+        self.assertEqual(list(self.module.merge()), [])
+
     def test_merge_does_not_suppress_index_error(self):
         # Issue 19018: Heapq.merge suppresses IndexError from user generator
         def iterable():