A waveguide diplexer is introduced where optimized low-insertion-loss metal insert filters are directly integrated in the E-planes of the T-junction arms. The filters are designed by the exact method of field expansion into suitable eigenmodes which takes the influences of higher-order-mode interaction and finite thickness of the inserts into account. Computer optimized design data are given which provide a metal-etching technique for reliable low-cost production. Measured minimum passband insertion losses of an R120-waveguide diplexer prototype are about 0.8 dB at 10.4 GHz, and 0.5 dB at 11.2 GHz. The stopband attenuation at 10.8 GHz is about 48 dB.