diff --git a/libtmux/pane.py b/libtmux/pane.py
index cfd943aa8..10498731e 100644
--- a/libtmux/pane.py
+++ b/libtmux/pane.py
@@ -104,17 +104,20 @@ def reset(self):
 
         self.cmd('send-keys', '-R \; clear-history')
 
-    def split_window(self, attach=False):
+    def split_window(self, attach=False, vertical=True):
         """Split window at pane and return newly created :class:`Pane`.
 
         :param attach: Attach / select pane after creation.
         :type attach: bool
+        :param vertical: split vertically
+        :type vertical: bool
         :rtype: :class:`Pane`.
 
         """
         return self.window.split_window(
             target=self.get('pane_id'),
-            attach=attach
+            attach=attach,
+            vertical=vertical
         )
 
     def set_width(self, width):
diff --git a/libtmux/window.py b/libtmux/window.py
index 0ab130731..f0b8e8a6e 100644
--- a/libtmux/window.py
+++ b/libtmux/window.py
@@ -346,7 +346,8 @@ def split_window(
             self,
             target=None,
             start_directory=None,
-            attach=True
+            attach=True,
+            vertical=True
     ):
         """Split window and return the created :class:`Pane`.
 
@@ -372,6 +373,8 @@ def split_window(
         :type start_directory: string
         :param target: ``target_pane`` to split.
         :type target: bool
+        :param vertical: split vertically
+        :type vertical: bool
 
         :rtype: :class:`Pane`
 
@@ -390,6 +393,11 @@ def split_window(
         else:
             tmux_args += ('-t%s' % self.panes[0].get('pane_id'),)
 
+        if vertical:
+            tmux_args += ('-v',)
+        else:
+            tmux_args += ('-h',)
+
         tmux_args += (
             '-P', '-F%s' % ''.join(tmux_formats)  # output
         )
diff --git a/tests/test_window.py b/tests/test_window.py
index fe9ee94b7..2643dc225 100644
--- a/tests/test_window.py
+++ b/tests/test_window.py
@@ -100,12 +100,23 @@ def test_attached_pane(session):
 
 
 def test_split_window(session):
-    """Window.split_window() splits window, returns new Pane."""
+    """Window.split_window() splits window, returns new Pane, vertical. """
     window_name = 'test split window'
     window = session.new_window(window_name=window_name, attach=True)
     pane = window.split_window()
     assert len(window.panes) == 2
     assert isinstance(pane, Pane)
+    assert float(window.panes[0].height) <= ((float(window.width) + 1) / 2)
+
+
+def test_split_window_horizontal(session):
+    """Window.split_window() splits window, returns new Pane, horizontal.  """
+    window_name = 'test split window'
+    window = session.new_window(window_name=window_name, attach=True)
+    pane = window.split_window(vertical=False)
+    assert len(window.panes) == 2
+    assert isinstance(pane, Pane)
+    assert float(window.panes[0].width) <= ((float(window.width) + 1) / 2)
 
 
 @pytest.mark.parametrize("window_name_before,window_name_after", [