Cofactor may refer to any of the following:
- Cofactor (linear algebra), the signed minor of a matrix
- Minor (linear algebra), an alternative name for the determinant of a smaller matrix than that which it describes
- Cofactor (biochemistry), a substance that needs to be present in addition to an enzyme for a certain reaction to be catalysed
- Shannon expansion or Shannon cofactor, a derived function of another function, a notion useful in computer science and mathematics