Photonic integrated circuits (PICs) are rapidly evolving toward extreme miniaturization, multi-wavelength operation, and multifunctional integration. Driven by breakthroughs in GPU-accelerated computation and topology optimization, inverse design has emerged as a powerful paradigm that overcomes the limitations of conventional geometry-driven approaches, resolving longstanding performance bottlenecks. In this work, we demonstrate an on-chip ultracompact broadband Fabry-Pérot (F-P) cavity enabled by inverse-designed reflectors, fabricated on a standard 220-nm silicon-on-insulator (SOI) platform. The reflector occupies a footprint of merely 2 µm × 2 µm and achieves an average reflectivity of ~ 0.97 across the 1500–1600 nm telecom window, enabling a high-loaded quality factor F-P resonator. Our inverse design strategy successfully reconciles the traditionally opposing demands of device miniaturization and broadband operation, while simultaneously demonstrating excellent robustness against nanofabrication tolerances. These results provide a scalable pathway toward high-density integration of advanced functionalities, including narrowband filtering, on-chip spectrometry, and engineered quantum light sources, within next-generation silicon photonic systems.