A sister school is usually a pair of schools, usually single-sex school, one with female students and the other with male students. This relationship is seen to benefit both schools. For instance, when Harvard University was a male-only school, Radcliffe University was its sister school. The sister school concept as a single-sex school began to change as several institutions adopted coeducational environments starting in the 1970s due to the increasing awareness or consciousness about sex bias and discrimination.

The term sister school (or brother school) has several alternate meanings: