A trimodal polymer waveguide interferometer was fabricated for bio/chemical sensing. Dual single-mode waveguides were used for the input and output ports to improve coupling efficiency in the trimodal waveguide. The performance of the interferometer was characterized via numerical simulation. Based on the analysis results, the optimum dimensions of the waveguide structures were determined, and the interferometer was actually fabricated using electron-beam lithography. The obtained results agreed well with those obtained via simulation. To demonstrate the applicability of the interferometer in biosensing, detection of glucose was conducted. A linear relationship between the phase change and glucose concentration was observed, which suggests potential applications of the TriMW interferometer in bio/chemical sensing.