Graduation portrait of Linus Pauling, 1922

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A mortarboard is an item of academic headgear consisting of a horizontal square board fixed upon a skull-cap, with a tassel attached to the centre. It is often termed trencher in the UK and Australia; in the U.S., it is usually referred to more generically as a cap. Its name derives from its resemblance to the board upon which mortar is placed prior to the laying of bricks. The cap, together with the gown and (sometimes) the hood, form the official uniform of a high school or university graduate.

The mortarboard is believed to have evolved from the biretta, a similar-looking hat worn by Christian clergy -- both are derivative of the Roman pileus quadratum. It was originally reserved for holders of masters degrees, but was later adopted by bachelors and undergraduates. Doctorate-holders of some universities wear the mortarboard, although the round bonnet is more common in Britain while the 4, 6, or 8 cornered "tam" is on ascendency in the U.S. In the U.S., the mortarboard is also worn by high school graduates during the presentation of their diplomas.

Until the second half of the 20th century, mortarboards were often worn by schoolteachers, and the hat remains an icon of the teaching profession. It is seen most often in comic representations of teachers, for example in the Bash Street Kids cartoon strip.

See also Academic dress.