An embodiment millimeter wave diplexer includes a substrate integrated waveguide (SIW) high pass filter (HPF), a microstrip line low pass filter (LPF), and a T-junction. The SIW HPF is coupled to a first port, and the microstrip line LPF is coupled to a second port. The SIW HPF is operable in a first frequency band, and the microstrip line LPF is operable in a second frequency band. The T-junction is coupled between the SIW HPF and the microstrip line LPF. The T-junction is also coupled to a common port.