Single-sex education, also known as single-gender education, same-sex education, same-gender education, selective-gender education, and gender-isolated education, is the practice of conducting education with male and female students attending separate classes, perhaps in separate buildings or schools. The practice of single-sex schooling was common before the 20th century, particularly in secondary and higher education.

Single-sex education is practiced in many parts of the world based on tradition and religion. Single-sex education is most popular in English-speaking countries (regions) such as Singapore, Malaysia, Ireland, the United Kingdom, Hong Kong, South Africa and Australia; also in Chile, Israel, South Korea and in many Muslim majority countries. In the Western world, single-sex education is primarily associated with the private sector, with the public (state) sector being overwhelmingly mixed sex; while in the Muslim world public schools and private schools are sex-segregated.

Motivations for single-sex education range from religious ideas of sex segregation to beliefs that the sexes learn and behave differently. As such, they thrive in a single-sex environment. In the 19th century, in Western countries, single-sex girls' finishing schools, and women's colleges offered women a chance of education at a time when they were denied access to mainstream educational institutions. The former was especially common in Switzerland, the latter in the U.S. and the U.K., pioneers in women's education.

In Western Europe before the 19th century, the most common way for girls to access education was at home, through private tutoring, and not at school, due to the strong resistance to women's involvement in schools. This attitude began to change in the 17th and 18th centuries, when girls' schools were established in both Catholic Europe, where they were managed by nuns, as well as in Protestant Europe, where they were managed by governesses, philanthropists, and private entrepreneurs. The development was similar in the U.S., where early feminists also successfully established women's educational institutions. These were different from and considered inferior to men's institutions. However, they created some of the first opportunities to formalized higher education for women in the Western world. The Seven Sisters colleges offered unprecedented emancipation for women. The pioneer Salem College of Winston-Salem, North Carolina, was founded in 1772, originally as a primary school, later becoming an academy (high school) and finally a college. The New England Female Medical College (1848) and the Woman's Medical College of Pennsylvania (1850) were the first medical institutions in the world established to train women in medicine and offer them the M.D. degree.

During the 19th century, ideas about education started to change: modern ideas that defined education as a right, rather than as a privilege available only to a small elite, started to gain support in North America and Europe. Mass elementary education was introduced, and more and more coeducational schools opened. Together with mass education, coeducation became standard in many places. Increased secularization in the 20th century also contributed to the acceptance of mixed sex education. In 1917 coeducation was mandated in the Soviet Union. According to Cornelius Riordan, "By the end of the nineteenth century, coeducation was all but universal in American elementary and secondary public schools (see Kolesnick, 1969; Bureau of Education, 1883; Butler, 1910; Riordan, 1990). Furthermore, by the end of the 20th century, this was largely true across the world. In the U.K., Australia, and Ireland, the tradition of single-sex education remained quite strong until the 1960s. The 1960s and 1970s were a period of intense social changes. Many anti-discrimination laws were passed during that era, such as the 1972 Title IX. Wiseman (2008) shows that by 2003, only a few countries globally have greater than one or two percent single-sex schools. But there are exceptions where the percent of single-sex schools exceeds 10 percent: Belgium, Chile, Singapore, the United Kingdom, Hong Kong, Israel, New Zealand, Australia, South Korea, and most Muslim nations. Recently, however, there has been a resurgence of interest in single-sex schools in modern societies across the globe, both in the public and private sector (Riordan, 2002)."

The topic of single-sex education is controversial. Advocates argue that it aids student outcomes such as test scores, graduation rates, and solutions to behavioral difficulties. Opponents, however, argue that evidence for such effects is inflated or non-existent and instead argue that such segregation can increase sexism and impairs the development of interpersonal skills.

Advocates of single-sex education believe that there are persistent sex differences in how males and females learn and behave in educational settings and that such differences merit educating them separately. One version of this argument holds that male-female brain differences favor implementing sex-specific teaching methods, but such claims have not held up to rigorous scrutiny. In addition, supporters of single-sex education argue that by segregating sexes, students do not become distracted by another sex's actions in the classrooms. Supporters of single-sex education also argue that the culture of coeducational settings causes some students to focus more on socialization, rather than prioritizing academics. Single-sex education supporters blame this focus on socialization for causing problems in student participation, attendance levels, and disciplinary problems.

A systematic review published in 2005 covering 2221 studies was commissioned by the United States Department of Education entitled Single-Sex Versus Coeducational Schooling: A Systematic Review. The review, which had statistical controls for socio-economic status of the students and resources of the schools, etc., found that in the study on the effects of single-sex schooling: