This work demonstrates an integrated multimode interferometer (MMI) based on a fully polymeric platform and optimized for visible range operation. The dimensions of a 2×2 MMI are first calculated analytically and simulated using finite elements method. The devices are manufactured using two layers of negative tone photoresists. The top layer is patterned by e-beam lithography demonstrating the adaptability of this material, naturally designed to respond to UV radiation. Fabrication tolerance was smaller than 100 nm. Devices were optically characterized with a 635 nm input source and the best performance for a 3 dB power splitter was found at an interferometric cavity dimension of 10.5 × 190.68 µm. Other interferometric lengths were characterized to establish a process design kit that allows future use of this platform in more complex photonic integrated circuits architectures.