We design an ultracompact, broadband, and low-loss 90-deg optical hybrid coupler using a silicon-on-insulator (SOI) material platform for coherent receivers in optical communication systems. The proposed hybrid coupler uses a cascaded topology that includes five 2  ×  2 multimode interference (MMI) couplers in three different fundamental kinds. The working principle of the suggested 90-deg optical hybrid is based on theoretical analysis on the MMI effect and the transfer matrix relations. The optimization of geometrical parameters and the optical characteristics are employing through the numerical simulation method. The investigated 90-deg optical hybrid coupler can be integrated on a microscale footprint as much as 6  μm  ×  172  μm. Furthermore, our hybrid coupler expresses lots of advantages of optical performances, with an insertion loss lower than 2.6 dB, a common-mode rejection ratio better than −25  dB, and phase error smaller than 5 deg in 90-nm wavelength-bandwidth in the third telecom window. In addition, the designed device, based on the SOI platform, is endowed with large geometrical tolerances possessing width, height, and multimode tolerances corresponding to ±20, ±10, and ±10  nm in the 1-dB variation limit of transmission, respectively. Such advantages of good performances thus make the proposed hybrid device playing the role of a promising potential candidate for widely varying applications of silicon photonics, such as coherent optical receivers, optical interconnects, and high-bitrate optical phase modulators.