We propose a refractive index sensor in a silicon-on-insulator (SOI) platform inspired by the operation of multimode fiber sensors. The sensor utilizes an optimized SOI resonator through adjoint-based inverse design topology optimization. The device’s refractive index distribution is calculated with a suitable figure of merit tailored for telecommunication band operation (1450 nm to 1650 nm) and is compatible with the standard fabrication processes. The flexibility of design offered through topology optimization and the localized interactions of the wave around the metastructure can be tailored to achieve maximum sensitivity. Our results may find interesting applications in wearable technologies, biosensing, and environmental monitoring.