A rear-seat armrest locking apparatus includes a release lever installed in an armrest to generate a pulling force, a link guide connected with the release lever through a release cable to provide a pulling force, and installed inside the armrest to move up and down, a link connected with the link guide to turn an up-and-down linear movement to a lateral movement, a locking pin connected with the link to laterally move by means of the pulling force transferred from the link, and being released from a locking hole of a side fixing plate, and a spring installed on the locking pin to return the locking pin to an original position of the locking pin and insert the locking pin into the locking hole. The rear-seat armrest locking apparatus prevents the armrest from being unfolded forward in a forward collision, thereby preventing an accident.