A bicycle speed sensor is provided on a bicycle (1). The bicycle (1) includes a chain stay (2), a rear wheel (3), a brake disk (4) and a positioning member (5). The rear wheel (3) includes a hub (3a) and the brake disk (4) includes a center hole (4a) disposed for the hub (3a) to pass through. The positioning member (5) passes through the chain stay (2), the center hole (4a) and the hub (3a) so that the rear wheel (3) and the brake disk (4) are positioned on the chain stay (2). The bicycle speed sensor includes a signal transmitter (10) and a signal sensor (20). The signal transmitter (10) is configured to transmit a signal and disposed on the brake disk (4). The signal sensor (20) receives the signal from the signal transmitter (10) and is disposed on the chain stay (2). By fixedly positioning the signal transmitter (10) and the signal sensor (20) at the brake disk and a default position of and the chain stay respectively, displacements of sensor components is prevented to provide reliable sensing data.