Modular cooling water assemblies for combined cycle power plant systems are disclosed. Each of the assemblies may include a pump and a heat exchanger, both positioned on a support structure. Additionally, each of the assemblies may include a closed cooling water system supported by the support structure. The closed cooling water system may include a return header fluidly coupled to a return conduit for receiving cooling water previously utilized by the combined cycle power plant system. The closed cooling water system may also include a supply header positioned adjacent the return header, and fluidly coupled to a supply conduit for supplying the cooling water to the combined cycle power plant system. Additionally, the closed cooling water system may include a closed cooling water circuit fluidly coupling the return header to the supply header. The closed cooling water circuit may be fluidly coupled to the heat exchanger and the pump.