An optical sensor employed in time-of-flight distance detection which avoids undesired light affecting the integrity of distance measurement includes a substrate, a cover, a light emitter, a measurement photodetector, and an optical blocking device. The cover is connected with the substrate and forms an internal space. The cover has a protrusion, a first light transmitting portion, and a second light transmitting portion. The protrusion extends toward the substrate and has a bottom, and divides the internal space into an interconnected first chamber and second chamber. The light emitter is arranged on the substrate and located in the first chamber. The measurement photodetector is disposed on the substrate and located in the second chamber. The optical blocking device is connected to the substrate and extends toward the cover and beyond the bottom of the protrusion.