A high isolation RF switch for launching a selected one of two RF signals into a slabline transmission line structure has a three switch configuration—a pair of input switches coupled to corresponding input ports and a transition switch coupled to an output port via a slabline transmission line. The input switches select one of the input ports for coupling to the transition switch, and the transition switch couples the selected input port from the input switches to the output port. A grounded conductor is situated between the input ports in a cavity of a housing within which the high isolation RF switch is located to provide isolation between the input ports. Additional isolation between the input ports is provided by coupling the non-selected RF signal to ground, either by grounding the non-selected input to the transition switch or the non-selected input port.