A socket wrench (or socket spanner) is a type of spanner (or wrench in North American English) that uses a closed socket format, rather than a typical open wrench/spanner to turn a fastener, typically in the form of a nut or bolt.

The most prevalent form is the ratcheting socket wrench, often informally called a ratchet. A ratchet incorporates a reversible ratcheting mechanism which allows the user to pivot the tool back and forth to turn its socket instead of removing and repositioning a wrench to do so.

Other common methods of driving sockets include pneumatic impact wrenches, hydraulic torque wrenches, torque multipliers and breaker bars. Some lesser known hybrid drivers include striking wrench tools with square drive, and hydraulic impact wrenches (typically powered by on site hydraulic power such as present with military tanks, and many rail car applications).

The basic contemporary form of socket is hexagonal, referred to as "6-point" for the pointed intersections where its six solid sided facets meet. These are attached to the driving tool via a male/female square connection fitting (called the square drive). Standard sizes of square drives around the world include 1⁄4, 3⁄8, 1⁄2, 3⁄4, 1, 1+1⁄2, 2+1⁄2 and 3+1⁄2 in (6.4, 9.5, 12.7, 19.1, 25.4, 38.1, 63.5 and 88.9 mm) square drive sizes (a de facto international standard with no metric equivalents) along with some lesser used drivers such as 5⁄8-inch square drive, and both No. 4 and No. 5 spline drives specified in ANSI B107 specifications. This wide range of square drive sizes provides for a wide variety of socket types and sizes to suit small to very large nuts and bolts. Some square drivers have a through hole to attach the socket to the driver (using a retaining ring with O-ring and pin type, or single piece molded retaining rings), a locking pin, or friction ball. Some common hand ratchets employ a quick release button on their top for quick socket release of smaller sockets. The tool chosen to drive the socket wrench ultimately supplies the mechanical advantage needed by the user to provide the torque needed to loosen or tighten the fastener as may be required. Larger drivers are typically used with higher torque, while smaller drivers are used for convenience in smaller low torque applications. Given the limits of human strength and fatigue, torque above 800 N⋅m (600 lbf⋅ft) of torque will generally involve some kind of power assist, instead of the user simply pushing on the handle of a wrench. Very large sockets and drivers are typically powered by hydraulics to achieve torque.

Male drivers are also produced for use with socket head cap screws, and are often called Allen drivers (trademark) or the generic term male bit drivers.

The principal advantage of interchangeable sockets is that, instead of a separate wrench for each of the many different fastener sizes and types, only separate sockets are needed for each size and type. Because of their versatility, nearly all screw and bolt types now have sockets of different types made to fit their bolts or nuts. Sockets often come as a "socket set" with many different sizes or types of sockets to fit the heads of different-sized fasteners. A ratchet of the "set size" is often included with the socket set. Sockets are commonly available in fractional inch and metric sizes, and in short (shallow) and longer (deep) varieties.