Rapid palatal expander which comprises an actuator element (2) provided with a drive head (3) and two sterns (4) with opposite threads, as well as a first and a second main body (5, 6) provided with aligned female threads (7, 8), each, engaged by a threaded stern (4) of the actuator element (2). Guide means (9) are provided for guiding the simultaneous movement of the main bodies (5, 6) along the longitudinal extension direction Y of the actuator element (2) following the rotation of the drive head (2) and anti-rotation means for preventing the rotation of the drive head (3) in the second rotation sense opposite the first rotation sense to close the expander (1). Such anti-rotation means comprise a plurality of longitudinal seats (61) obtained on the peripheral surface (3A) of the drive head (3) and each having an abutment face (62) at least partially directed in the sense of the second rotation sense. In addition, the anti-rotation means also comprise a plate (64) mechanically and rigidly connected to one of the two main bodies (5, 6) and provided with a flexible tab (65) elastically in abutment against the peripheral surface (3A) of the drive head (3) for the entire adjustment travel of the main body (5, 6), being able to be selectively engaged with its profile (65′) in the longitudinal seats (61) of the head (3) at angular adjustment positions assumed by the latter along the adjustment travel.
