A lap joint or overlap joint is a joint in which the members overlap. Lap joints can be used to join wood, plastic, or metal. A lap joint can be used in woodworking for joining wood together.

A lap joint may be a full lap or half lap. In a full lap, no material is removed from either of the members that will be joined, resulting in a joint which is the combined thickness of the two members. In a half lap joint or halving joint, material is removed from both of the members so that the resulting joint is the thickness of the thickest member. Most commonly in half lap joints, the members are of the same thickness and half the thickness of each is removed.

With respect to wood joinery, this joint, where two long-grain wood faces are joined with glue, is among the strongest in ability to resist shear forces, exceeding even mortise and tenon and other commonly-known "strong" joints.

With respect to metal welding, this joint, made by overlapping the edges of the plate, is not recommended for most work. The single lap has very little resistance to bending. It can be used satisfactorily for joining two cylinders that fit inside one another.