A modular expandable weighting system for recreational and professional underwater divers that permits a user to selectively attach one or more individual weight units to each other to achieve a desired amount of weight to regulate the user's overall buoyancy while under water. When used with a weight belt, the weights when attached create channels through which a weight belt may be disposed. If two or less weights are attached, end caps may be attached to create the channels to allow the system to be attached to the weight belt. When used without a weight belt, such as on a buoyancy compensator device with multiple weight pockets, the weight system allows for the placement of exact amounts of weights to achieve the desired amount of overall buoyancy as well as to maintain the desired degree of horizontal trim.