The back-up wrench comprises a stationary jaw having an angular gripping wall adapted to grip two adjacent sides of different size connectors, and a rotatable jaw having a concave wall adapted to grip limited portions of two symmetrically-opposite sides of the connectors. The rotatable jaw is pivotally mounted on a cylindrical bearing surface provided by a forward extension of a plate whose rearward end is pivotally mounted on a stationary handle forming integral part with the stationary jaw. A rotatable handle is pivotally coupled to the plate. Conventional means are provided in and between the handles for applying and releasing the gripping action of the jaws against the four sides of the connector.