Abstract: An adapter for selectively connecting an accessory to a spray gun has an upstream connector for connecting to the spray gun, a downstream connector for connecting to accessory, a gasket located between the upstream and downstream connectors, and the upstream and downstream connectors are selectively connected together. When connected, one of the upstream and downstream connectors has a cylindrical portion at least partially inserted into another one of the upstream and downstream connectors, and a post projecting radially from the cylindrical portion. The other one of the upstream and downstream connectors has an arcuate slot being shaped and structured to receive the post of the one of the upstream and downstream connectors. A spray gun assembly has spray gun, an accessory and at least one adapter connecting the spray gun and the accessory together.