A caponier is a type of defensive structure in a fortification. Fire from this point could cover the ditch beyond the curtain wall to deter any attempt to storm the wall. The word originates from the French caponnière, meaning "chicken coop" (a capon is a castrated male chicken).

In some types of bastioned fortifications, the caponier served as a means of access to the outworks, protecting troops from direct fire; they were often roofless. Although they could be used for firing along the ditch, the flanks of the bastions were the main defence of the ditch by fire.

In later polygonal forts, caponiers were often roofed, and were not intended as a type of covered way, but as the main way of keeping the ditch clear of the enemy.