Many studies have indicated that single-sex classrooms promote female students’ self-confidence and achievement in various professions, including professions where their rate of employment is low, such as mathematics and other exact sciences. The purpose of the present study is to examine the relationship between single-sex classrooms and math achievements among both female and male students. The study population included 608 students learning in the fifth-ninth grades, who attended state-religious schools in the southern region of Israel and came from families who had similar socioeconomic status. The students answered a short demographic questionnaire and their math teacher filled out each student’s score achieved in the regional math summative assessment. The findings showed no relationship between classroom type and boys’ achievement in mathematics, while a significant relationship was found between learning in single-sex classroom and higher math achievements among girls in elementary school. In middle school, however, no significant difference was found.