A carriage bolt, coach bolt or round head square neck boltis a form of bolt.

It is distinguished from other bolts by its head: the shank of the bolt is circular for most of its length, as usual, but the portion immediately beneath the head is formed into a square section.This makes the bolt self-locking when placed through a square hole, or a round hole in most wood. This allows the fastener to be installed with only a single tool, a spanner or wrench, working from one side. The head of a carriage bolt is usually a shallow dome. The squared section is of the same size as the diameter of the bolt shank, with a plain unthreaded shank.

Carriage bolts were developed for use through iron strengthening plates on either side of a wooden beam. It is commonplace though to use them into bare timber, the squared section giving enough grip to prevent rotation.