A yarn sensor for textile spinning and bobbin-winding machines for optically scanning a longitudinally traveling yarn (3) within a measurement gap (19), particularly for detecting extraneous fibers, includes a light source (20), a first receiver (23) for directly transmitted light, two further receivers (21, 22) for light reflected from the yarn (3), and light transmitting elements (24, 25, 26, 27) between the light source (20), measurement gap (19) and receivers (21, 22, 23). In the absence of the yarn (3), the receivers (21, 22) detect projected images of the opposite wall (62) of the measurement gap (19), located essentially outside both sides of the area of the wall (62) of the measurement gap (19) illuminated by the direct radiation of the light-emitting diode (20). As a result, parasitic signals are suppressed, improving the detection of extraneous fibers.