21st Battalion

21st Battalion may refer to:


 * 21st Battalion (Australia), a World War I ANZAC battalion
 * 2/21st Battalion (Australia), a World War II Australian infantry battalion
 * 21st Battalion (New Zealand), a World War II New Zealand infantry battalion
 * 21st Battalion (Eastern Ontario), CEF, a World War I battalion that formed part of the Canadian Corps
 * 21st Battalion, London Regiment (First Surrey Rifles)