A windscreen wiper (1) is described, of the type in which the wiper blade support arm (4) is mounted slidable on an arm support member (7) which is rotated alternately in opposite directions by a rotatable pin (3) of a drive motor (2); the support member (7) is mounted idly about the pin (3) and receives its motion from this latter by way of a transmission comprising gears (12,16,18) and a pinion (14) rigid with said pin (3); the wiper blade support arm (4) is moved by a bar (20) hinged to it and provided with an internally toothed track (30) defining an endless path comprising two parallel rectilinear branches which are joined together by curved portions, and are selectively engaged by said pinion (14) in order to move the bar (20) relative to this latter.