A semiconductor photodetector includes a semiconductor substrate of a first conductivity type, a light absorption layer of the first conductivity type on the semiconductor substrate and absorbing light, a diffraction grating layer on the light absorption layer and including a diffraction grating diffracting light, a first light transmissive layer of a second conductivity type on the diffraction grating layer and transmitting light, and a second light transmissive layer of the first conductivity type on the diffraction grating layer and surrounding the first light transmissive layer, the second light transmissive layer transmitting light. The diffraction grating surrounds a region of the diffraction grating layer that is directly below the first light transmissive layer.