31st Battalion

31st Battalion may refer to:


 * 31st Battalion, Royal Queensland Regiment, a unit of the Australian Army
 * 31st/42nd Battalion, Royal Queensland Regiment, a unit of the Australian Army
 * 31st (Alberta) Battalion, CEF, a unit of the Canadian Army
 * 31st Infantry Battalion (Estonia), a unit of the Estonian Army
 * 31st Engineer Battalion, a unit of the United States Army
 * Combat Logistics Battalion 31, a unit of the United States Marine Corps