A magnetorheological actuator including a chamber, in particular a chamber at least sections of which are shaped like a hollow cylinder, containing a magnetorheological fluid, and further including a control element disposed in the chamber, wherein the control element suitable for applying the operating force of the actuator, wherein the control element is a threaded spindle or screw or threaded rod driven to rotate about its longitudinal axis, in particular about the screw axis, and including a spindle core and a thread disposed thereon, the thread surrounded by the magnetorheological fluid, is provided. A clutch having such an actuator is also provided.