We present a method for formulating the design problem of maximizing the transduction efficiency of a magnetostrictive patch-type transducer as a topology optimization problem. Unlike existing methods based on magnetic analysis alone, our method is based on a coupled magnetomechanical analysis. It employs a quasi-static magnetomechanical transduction model relating applied magnetic field and induced stress in a magnetostrictive patch to facilitate design optimization. For the generation of a specific elastic wave mode, not only the patch shape but also the state of patch-waveguide bonding should be optimized simultaneously. We therefore developed a modified topology optimization formulation. The optimal results are consistent with physical intuition and some existing experimental findings for the case of torsional and longitudinal waves in a cylindrical waveguide.