A device for measuring the diameter and/or the wall thickness of a strand that has a substantially circular cross-section and is guided through the device by guide means in the direction of its longitudinal axis includes at least one transmitter for transmitting terahertz radiation, at least one radiation optical system that conducts the terahertz radiation to a strand guided by the device, at least one reflector for the terahertz radiation arranged opposite a transmitter and behind the strand in the radiation direction of the terahertz radiation, at least one receiver for receiving the terahertz radiation reflected at the strand and/or the reflector, and an evaluation apparatus that determines the diameter and/or the wall thickness of the strand using the measuring signals received by the at least one receiver. A corresponding method is also described.