A waterproof battery includes a casing having an opening; a battery body received in the casing; a connecting unit disposed in the casing, wherein the connecting unit has a circuit board electrically connected to the battery body and a connector electrically connected to the circuit board and protruded from the opening; and a waterproof adhesive layer for being affixed to and thereby connecting an inner surface of the casing and a bottom side of the circuit board so as to hermetically seal the opening. Accordingly, the waterproof battery is capable of preventing water or liquid from intruding into the battery body to cause damage thereto, that is, a built-in capacity to be water-free.