The integrated optical 90-degree hybrid is a crucial component for coherent receivers. Here, we simulate and fabricate a 4?×?4 multimode interference coupler as a 90-degree hybrid using thin film lithium niobate (TFLN). The device features low loss (0.37?dB), high common mode rejection ratio (over 22?dB), compact footprint, and small phase error (below 2°) within the whole C-band experimentally, which is promising for integration with coherent modulators and photodetectors for TFLN-based high-bandwidth optical coherent transceivers.