A snap-on battery includes a battery body and a fastening element, and the battery body has a guiding element disposed on a side of the battery body, and the fastening element is slidably coupled to and positioned at the guiding element, and the fastening element has a combining portion for combining an external connecting element. With the aforementioned components, the battery can be detachably snapped on a portable electronic device, and even if a user forgets to close a door with cover or secure the door with cover, the battery will be still hard to slip off the portable electronic device or damages due to falling out.