An optoelectronic sensor, especially a reflection photoelectric barrier and a reflection light sensing device, for detection of an object (2) in a monitored area, with a housing (3) with transmitting and receiving optics (6) and with an evaluation circuit (10). The optoelectronic sensor has an especially simple structure in that there is only one single optoelectronic transmitting and receiving component (14) which sequentially acts as both the opto-transmitter and the opto-receiver in succession in time.