An adjustable wrench includes a handle having an end on which a fixed jaw is formed. The handle includes an engaging section and a transverse through-hole. A movable jaw includes a longitudinal hole through which the engaging section of the handle extends, allowing the movable jaw to slide relative to the handle. A retaining member is movably mounted in the transverse through-hole of the handle and biased by an elastic element to a position where toothed sections of the retaining member are engaged with toothed sections of the movable jaw, thereby retaining the movable jaw in place. The retaining member can be pushed to another position wherein the toothed sections of the retaining member are disengaged from the toothed sections of the movable jaw, allowing the movable jaw to slide relative to the fixed jaw.