Abstract: A modular valve assembly of this invention includes individual valve units and components, which can be selectively combined to form an integrated multi-port, multi-way valve assembly that is operated by a single common linear actuator is disclosed. The modular design of this invention is facilitated by the construction of valve housings and valve components of the individual valve units and assemblies of common plumbing pipes and fittings, such as, tee joints, threaded couplings, bushings and reducers. A single linear actuator, such as a conventional solenoid or piston cylinder mounted to the end of the valve housing, reciprocates an elongated shaft running axially through the valve interior to move various valve discs into and out of engagement with various internal valve seats, thereby controlling the valve assembly operation.