A pump assembly for a dishwashing appliance includes an inner diffuser shell defining a motor housing and an outer diffuser shell spaced apart from the inner diffuser shell to define a diffusing plenum. A motor positioned within the motor housing rotates a wash pump impeller and urges a flow of wash fluid through the diffusing plenum. Within the diffusing plenum, guide vanes extend between the inner diffuser shell and the outer diffuser shell to diffuse and direct the flow of wash fluid.