Seisen International School (SIS) is an all-girls Catholic school located in Setagaya, in Tokyo, Japan. Students and faculty from over sixty countries are represented at Seisen, with English being the main language of instruction. It offers the International Baccalaureate Diploma Programme for grades 11 and 12. It is accredited by the Ministry of Education, Culture, Sports, Science and Technology (Japan), New England Association of Schools and Colleges and the Council of International Schools. It teaches Montessori kindergarten (coed, ages 2–6) and grades 1-12 (girls only, ages 6–18).

According to the Good Schools Guide International, "The school is popular and has a good reputation."

According to Anne-Marie De Mejía, Seisen International School provides a specifically Christian orientation in its programmes.