An adjustable wrench includes a handle, a head, a slidable jaw, an adjusting assembly, and a reinforcing device. The head formed at one end of the handle and includes a fixed jaw, a sliding rail, a receiving slot, two lateral walls, and two connecting portions. The sliding rail is formed between the two lateral walls. The receiving slot communicates with the sliding rail. Each connecting portion is formed at one end of each lateral wall opposite to the fixed jaw. The slidable jaw is slidably disposed in the sliding rail. The adjusting assembly is positioned in the receiving slot and engaged with the slidable jaw. The reinforcing device connects the two lateral walls and is engaged into the two connecting portions.