This paper presents a topology optimization approach to design planar transitions between a microstrip line (MSL) and a rectangular waveguide (RWG) in the K-band. The transition comprises two sub-transitions: one from the MSL to a substrate integrated waveguide (SIW) and the second from the SIW to the RWG. Both are on the same substrate and can be manufactured with a standard printed circuit board process. This leads to a very costeffective solution compared with other approaches. A WR-42 waveguide can easily be surface mounted to the transitions using a standard flange. The transitions have been fabricated, and their measured performance shows good agreement with the simulations. The MSL-SIW transition has a broadband behavior and the SIW-RWG transition still reaches a relative bandwidth of 10%.