A monolithically integrated wavelength-selective photodetector operating at long wavelength was reported in this paper. The photodetector, which consists of an GaAs-based Fabry-Pérot filter and an InP-based p-i-n absorption structure, was grown on a Si substrate by heteroepitaxy technology. A crack-free and high-quality GaAs epilayer was obtained by using mid-patterned growth. The photodetector with a spectral linewidth of 1.1 nm (FWHM) and a quantum efficiency of 9.0% was demonstrated, and its absorption layer thickness is 300 nm.