Guillemot is the common name for several species of seabird in the Alcidae or auk family (part of the order Charadriiformes). In British use, the term comprises two genera: Uria and Cepphus. In North America the Uria species are called murres and only the Cepphus species are called "guillemots". This word of French origin derives from a form of the name William, cf. French: Guillaume.

The two living species of Uria, together with the razorbill, dovekie, and the extinct great auk, make up the tribe Alcini. They have distinctly white bellies, thicker and longer bills than Cepphus, and form very dense colonies on cliffs during the reproductive season. Guillemot eggs are large (around 11% of female weight), pyriform in shape, and colourful, making them attractive targets for egg collectors.

The three living species of Cepphus form a tribe of their own: Cepphini. They are smaller than the Uria species and have black bellies, rounder heads and bright red feet.