diff --git a/mapstructure.go b/mapstructure.go
index c5e33be..d3cb4e8 100644
--- a/mapstructure.go
+++ b/mapstructure.go
@@ -50,6 +50,11 @@
 	// (extra keys).
 	ErrorUnused bool
 
+	// ZeroFields, if set to true, will zero fields before writing them.
+	// For example, a map will be emptied before decoded values are put in
+	// it. If this is false, a map will be merged.
+	ZeroFields bool
+
 	// If WeaklyTypedInput is true, the decoder will make the following
 	// "weak" conversions:
 	//
@@ -438,9 +443,15 @@
 	valKeyType := valType.Key()
 	valElemType := valType.Elem()
 
-	// Make a new map to hold our result
-	mapType := reflect.MapOf(valKeyType, valElemType)
-	valMap := reflect.MakeMap(mapType)
+	// By default we overwrite keys in the current map
+	valMap := val
+
+	// If the map is nil or we're purposely zeroing fields, make a new map
+	if valMap.IsNil() || d.config.ZeroFields {
+		// Make a new map to hold our result
+		mapType := reflect.MapOf(valKeyType, valElemType)
+		valMap = reflect.MakeMap(mapType)
+	}
 
 	// Check input type
 	dataVal := reflect.Indirect(reflect.ValueOf(data))
diff --git a/mapstructure_test.go b/mapstructure_test.go
index 1c24e3f..e05dcc6 100644
--- a/mapstructure_test.go
+++ b/mapstructure_test.go
@@ -517,6 +517,38 @@
 	}
 }
 
+func TestMapMerge(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"vfoo": "foo",
+		"vother": map[interface{}]interface{}{
+			"foo": "foo",
+			"bar": "bar",
+		},
+	}
+
+	var result Map
+	result.Vother = map[string]string{"hello": "world"}
+	err := Decode(input, &result)
+	if err != nil {
+		t.Fatalf("got an error: %s", err)
+	}
+
+	if result.Vfoo != "foo" {
+		t.Errorf("vfoo value should be 'foo': %#v", result.Vfoo)
+	}
+
+	expected := map[string]string{
+		"foo":   "foo",
+		"bar":   "bar",
+		"hello": "world",
+	}
+	if !reflect.DeepEqual(result.Vother, expected) {
+		t.Errorf("bad: %#v", result.Vother)
+	}
+}
+
 func TestMapOfStruct(t *testing.T) {
 	t.Parallel()