An apparatus for locking a double-folding seat for vehicles includes a latch plate, a guide unit, a release lever unit and a locking pin. The latch plate has a locking notch therein and is fastened to a seat frame. The guide unit is fitted over a folding shaft of the seat frame at a position spaced apart from the latch plate. The release lever unit is provided on the folding shaft so as to be rotatable. The locking pin is elastically connected to the release lever unit such that the locking pin is locked to the locking notch of the latch plate when the seat frame is folded. Furthermore, when the release lever unit is rotated, the release lever unit pushes the locking pin, so that the locking of the locking pin to the locking notch of the latch plate is released.