The present disclosure provides an armrest-locking device that is disposed in an armrest that is stowed in an armrest case of a rear seatback. The armrest-locking device includes: a locking member disposed at any one of the armrest case and the armrest; a locking bracket rotatably coupled to the other one of the armrest case and the armrest, the locking member inserted into a groove formed on a peripheral portion of the locking bracket when the armrest is stowed; and a holding bracket. In particular, the locking bracket rotates in a forward direction to be fastened to the locking member when the locking member is inserted; and the holding bracket has an end to engage with the locking bracket when the locking bracket is rotated, maintaining the rotation state of the locking bracket.