A mortise and tenon (occasionally mortice and tenon) is a joint that connects two pieces of wood or other material. Woodworkers around the world have used it for thousands of years to join pieces of wood, mainly when the adjoining pieces connect at right angles, though it can be used to connect two work pieces at any angle.

Mortise-and-tenon joints are simple, strong, and stable, and can be used in many projects and which give an attractive look. They are either glued or friction-fitted into place. This joint is difficult to make, because of the precise measuring and tight cutting required; as such, modern woodworkers often use machinery specifically designed to cut mortises and matching tenons quickly and easily. Still, many woodworkers cut them by hand in a traditional manner. There are many variations of this type of joint, but its basic structure has two components, the mortise hole and the tenon tongue.

The tenon, formed on the end of a member generally referred to as a rail, fits into a square or rectangular hole cut into the other, corresponding member. The tenon is cut to fit the mortise hole exactly. It usually has shoulders that seat when the joint fully enters the mortise hole. The joint may be glued, pinned, or wedged to lock it in place.

This joint is also used with other materials, as traditionally by both stonemasons and blacksmiths.

Mortise, "a hole or groove in which something is fitted to form a joint," comes from c. 1400 from Old French mortaise (13th century), possibly from Arabic murtazz, "fastened", past participle of razza, "cut a mortise in." Tenon, in use in English since the late 14th century, developed its nounal sense of "a projection inserted to make a joint" from the Old French tenir "to hold".

The mortise-and-tenon joint is an ancient one. One of the earliest mortise-and-tenon structure examples dates back 7,000 years to the Hemudu culture in China's Zhejiang Province. Tusked mortise-and-tenon joints were found in a well near Leipzig, created by early Neolithic Linear Pottery culture, and used in construction of the wooden lining of the wells. Mortise-and-tenon joints have been found joining the wooden planks of the "Khufu ship", a 43.6 m (143 ft) long vessel sealed into a pit in the Giza pyramid complex of the Fourth Dynasty around 2500 BC. They were also found in the Uluburun shipwreck (14th century BC).

Mortise-and-tenon joints have also been found in ancient furniture from archaeological sites in the Middle East, Europe and Asia. Many instances are found, for example, in ruins of houses in the Silk Road kingdom of Cadota, dating from the first to the 4th century BC. In traditional Chinese architecture, wood components such as beams, brackets, roof frames, and struts were made to interlock with perfect fit, without using fasteners or glues, enabling the wood to expand and contract according to humidity. Archaeological evidence from Chinese sites shows that, by the end of the Neolithic, mortise-and-tenon joinery was employed in Chinese construction.

The thirty sarsen stones of Stonehenge were dressed and fashioned with mortise-and-tenon joints before they were erected between 2600 and 2400 BC.