A graywater diverter vessel assembly (as shown in FIG. 2) comprises a cylindrical side vessel 4 and valve diversion assembly 1, 2, 16. The device fits onto a vertical wastewater pipe 9, wherein a user can by closing the valve 1, divert graywater to upflow into the cylindrical side vessel 4 and pass through the vessel's removable mesh filter 5 and outflow point 7 for reuse, or alternatively by opening the valve permit graywater to flow downward through the valve diversion assembly and sewer or onsite wastewater system. The cylindrical side vessel has an over-flow outlet 8 so graywater can overflow and bypass the closed valve and flow directly to the sewer or onsite wastewater system in the event that the free outflow of graywater is restricted from either clogging of the mesh filter or other blockage, without restricting or limiting the continued normal free outflow of graywater from the building's plumbing fixtures.