A mortarboard is a special type of cap worn with academic regalia at graduations and in certain academic settings. Because mortarboards have an extremely distinctive appearance, they are often linked in the public eye with the academic world and with the celebrations associated with graduation. They have also historically been linked with the teaching profession, a throwback to an era when instructors wore full academic regalia, including mortarboards, while teaching.

A classic mortarboard consists of a cap which is attached to a hard, flat square of material. Typically, a tassel is worn with a mortarboard, with the tassel being attached with a button or tab in the middle of the square section of the mortarboard. Depending on the design, the cap may be soft or hard, and it may have laces or elastic to ensure a snug fit, as a properly fitting mortarboard should be tight.

Traditionally, the board of a mortarboard is positioned so that it is aligned to be parallel with the ground, while the bulk of the cap rests on the back of the head. Depending on regional traditions, the positioning of the tassel may be important; sometimes the location indicates whether or not the wearer has graduated, or what level of education the wearer has completed.

Classically, mortarboards are designed in the same color as the rest of the wearer's academic regalia, so they are typically black. In the case of colored regalia, the mortarboard may be the same color as the gown, or worn in a contrasting color to create a coordinated look which reflects the school colors of the graduate. The tassel's colors may be matched to the school colors, or they may reflect the discipline the student studied.

The origins of the mortarboard are quite old. In Roman times, a skullcap much like the modern mortarboard was worn, and such caps were also worn by Muslim scholars in the Middle East. By the Middle Ages, mortarboards were worn by members of some holy orders, which explains how they entered the educational system in Europe; most students were also members of religious orders in the Middle Ages, when the mortarboard flourished.

This cap goes by a number of alternate names, including corner cap, in a reference to the four corners of the square, trencher, or square cap. The slang term “mortarboard” is a reference to the easily carried boards used by masons to store mortar to work with as they move along on a project.