A locking structure for an armrest is provided to be pivotable to a folded up or a pulled down position. The locking structure includes a locking pin disposed at a fixed position and a catch pivotably coupled to a fixing bracket in an armrest housing, and has an engaging portion formed at a side thereof so that the locking pin enters the engaging portion, and a projection and a recess are formed to be spaced apart from the engaging portion. A latch is coupled to the fixing bracket and has a protruding portion formed at one side of the latch. A knob rotates the latch and when the armrest housing rotates upward, the catch rotates while the locking pin enters the engaging portion to lock rotation of the armrest housing. When the latch is, the locking pin is withdrawn from the catch, to permit the rotation of the armrest housing.