A spray gun includes a gun body, a rotary plug and a button. The gun body has a valve seat extending between an inlet passage and an outlet passage thereof. The plug is accommodated in a chamber of the valve seat and has a fluid path, with the plug rotatable between an open position where the fluid path joins the inlet passage and the outlet passage and a closed position where the plug separates the inlet passage from the outlet passage. The button has two lobes joined to opposite ends of the plug. An elastomeric seal ring is sleeved around the plug. A backup ring adjacent to the seal ring is sleeved around the plug. The seal ring and the backup ring are held between a shoulder of the valve seat and the respective lobe of the button.