St. John the Baptist Diocesan High School is a private, Roman Catholic high school in West Islip, New York.

It is operated by the Roman Catholic Diocese of Rockville Centre.

Faculty members at St. John's are required to possess a master's degree, and have New York State Teacher Certification.

Honors, Advanced Placement and regents courses.

Also, college credit courses are offered through St. John's affiliates such as St. John's University, Molloy College and the New York Institute of Technology.