Abstract: A universal wrench is described for tightening or loosening polygonal assembly elements, such as nuts, bolt heads or polygonal tube sleeves. This universal wrench includes a wrench body provided with a handle (1) and a wrench head (2) intended to come into contact with a polygonal assembly element (5) to be tightened or loosened, said wrench head including a support body (20) able to accommodate said polygonal assembly element. The wrench further includes a flexible strap-shaped element (3) intended to surround and hold the polygonal assembly element against the support body and tension means for the strap. The wrench head is also rotatably mounted with respect to the handle so as to allow an increase in the tension of the strap when a rotational movement is applied to the assembly element and to allow the strap to relax during an opposite movement.