In mathematics, a genus g surface (also known as a g-torus or g-holed torus) is a surface formed by the connected sum of g distinct tori: the interior of a disk is removed from each of g distinct tori and the boundaries of the g many disks are identified (glued together), forming a g-torus. The genus of such a surface is g.

A genus g surface is a two-dimensional manifold. The classification theorem for surfaces states that every compact connected two-dimensional manifold is homeomorphic to either the sphere, the connected sum of tori, or the connected sum of real projective planes.

The genus of a connected orientable surface is an integer representing the maximum number of cuttings along non-intersecting closed simple curves without rendering the resultant manifold disconnected. It is equal to the number of handles on it. Alternatively, it can be defined in terms of the Euler characteristic χ, via the relationship χ = 2 − 2g for closed surfaces, where g is the genus.

The genus (sometimes called the demigenus or Euler genus) of a connected non-orientable closed surface is a positive integer representing the number of cross-caps attached to a sphere. Alternatively, it can be defined for a closed surface in terms of the Euler characteristic χ, via the relationship χ = 2 − g, where g is the non-orientable genus.

An orientable surface of genus zero is the sphere S². Another surface of genus zero is the disc.

A genus one orientable surface is the ordinary torus. A non-orientable surface of genus one is the projective plane.

Elliptic curves over the complex numbers can be identified with genus 1 surfaces. The formulation of elliptic curves as the embedding of a torus in the complex projective plane follows naturally from a property of Weierstrass's elliptic functions that allows elliptic curves to be obtained from the quotient of the complex plane by a lattice.

The term double torus is occasionally used to denote a genus 2 surface. A non-orientable surface of genus two is the Klein bottle.