A heater for footwear includes a seat having a space centrally defined therein. A spring is compressively received in the space in the seat. A friction mechanism is received in the seat. A cap is sleeved on the seat. The cap has a driving unit longitudinally extending therefrom. The driving unit extends through the friction mechanism for movably abutting against the spring in the space in the seat. When the cap is pressed/released, the driving is unit moved relative to the seat for engaging the friction mechanism to warm up the footwear.