In geometry, a circumcircle of a given two-dimensional geometric shape is the smallest circle which contains the shape completely within it. Every bounded shape has a unique circumcircle. Some (but not necessarily all) vertices of the shape will lie on the circumcircle.

In particular, a polygon whose vertices all lie on its circumcircle is said to be a cyclic polygon. Every triangle is cyclic in that sense. The center of a triangle's circumcircle, also called its cirumscribed circle, is called its circumcenter.

See also incircle.