diff --git a/src/mame/atari/stkbd.cpp b/src/mame/atari/stkbd.cpp
index 84dc8a1beed..25ece09815b 100644
--- a/src/mame/atari/stkbd.cpp
+++ b/src/mame/atari/stkbd.cpp
@@ -49,6 +49,7 @@ st_kbd_device::st_kbd_device(const machine_config &mconfig, const char *tag, dev
 	m_joy(*this, "JOY%u", 0U),
 	m_mousex(*this, "MOUSEX"),
 	m_mousey(*this, "MOUSEY"),
+	m_mouseb(*this, "MOUSEB"),
 	m_config(*this, "config"),
 	m_keylatch(0),
 	m_mouse(0),
@@ -147,11 +148,18 @@ uint8_t st_kbd_device::port2_r()
 {
 	//  bit     description
 	//  0       JOY 1-5
-	//  1       JOY 0-6
-	//  2       JOY 1-6
+	//  1       JOY 0-6 / mouse button 1
+	//  2       JOY 1-6 / mouse button 2
 	//  3       serial from cpu
 
-	return (m_joy[1]->read() & 0x06) | (m_tx << 3);
+	uint8_t data;
+
+	if ((m_config->read() & 0x01) == 0)
+		data = m_mouseb->read();
+	else
+		data = m_joy[0]->read();
+
+	return (data & 0x06) | (m_tx << 3);
 }
 
 void st_kbd_device::port2_w(uint8_t data)
@@ -358,6 +366,10 @@ static INPUT_PORTS_START(stkbd)
 
 	PORT_START("MOUSEY")
 	PORT_BIT( 0xff, 0x00, IPT_MOUSE_Y ) PORT_SENSITIVITY(100) PORT_KEYDELTA(5) PORT_MINMAX(0, 255) PORT_PLAYER(1)
+
+	PORT_START("MOUSEB")
+	PORT_BIT( 0x02, IP_ACTIVE_LOW, IPT_BUTTON1) PORT_NAME("Mouse Button 1") PORT_CODE(MOUSECODE_BUTTON1)
+	PORT_BIT( 0x04, IP_ACTIVE_LOW, IPT_BUTTON1) PORT_NAME("Mouse Button 2") PORT_CODE(MOUSECODE_BUTTON2)
 INPUT_PORTS_END
 
 ioport_constructor st_kbd_device::device_input_ports() const
diff --git a/src/mame/atari/stkbd.h b/src/mame/atari/stkbd.h
index 052a47f0860..6a3c59c4c07 100644
--- a/src/mame/atari/stkbd.h
+++ b/src/mame/atari/stkbd.h
@@ -42,6 +42,7 @@ private:
 	required_ioport_array<2> m_joy;
 	required_ioport m_mousex;
 	required_ioport m_mousey;
+	required_ioport m_mouseb;
 	required_ioport m_config;
 
 	uint16_t m_keylatch;