An adjustable wrench is a tool used to tighten or loosen hexagonal bolts or nuts. The advantage over a standard wrench is that a small worm gear allows the wrench to be adjusted to fit a wide variety of sizes, both metric and imperial.

The adjustable wrench is also known as the "shifting spanner", or simply the "shifter". The disadvantage of a shifter is that it suffers from considerably more slippage than the conventional spanner. Thus 'shifters' are often responsible for sudden excruciating pain in the knuckles of the user, together with some loss of skin.