The Mode-Matching Technique (MMT) is deployed to analyze and design a Substrate Integrated Waveguide (SIW) T-junction diplexer in K-band. The diplexer has bandwidths of 2.75 percent and 2.63 percent at 18.15 GHz and 19 GHz, respectively. The MMT results are compared with simulated data obtained from commercially available field solvers such as CST Microwave Studio, μWave Wizard and ANSYS HFSS. Excellent agreement between MMT and simulated data is achieved. Measured data obtained from a fabricated prototype diplexer agree well with simulated and MMT results.