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)