St. Louis University High School (SLUH) is an all-male Jesuit high school in St. Louis, Missouri. Founded in 1818, it is the oldest secondary educational institution in the United States west of the Mississippi River, and one of the largest private high schools in Missouri. It is located in the Archdiocese of St. Louis.

SLUH was founded in 1818 by the bishop of St. Louis, Louis Dubourg, as a Latin school for boys known as St. Louis Academy. Classes were held in a one-story house owned by Madame Alvarez on the northwest corner of Third and Market Street. It quickly grew to include a college division, and the college was granted university status in 1832. The high school retained the identity of St. Louis Academy on the university campus until 1924 when it moved to its own facilities and incorporated separately under the name of St. Louis University High School. The school's new home, on Oakland Avenue, was a gift of Anna Backer in memory of her late husband and alumnus George Backer. That facility, also known as Backer Memorial, has grown considerably over the years and remains the school's home.

The high school integrated when it enrolled John Carter, a sophomore transfer from Saint Thomas Academy in St. Paul, Minnesota, in 1946, one year before Cardinal Joseph Ritter issued a pastoral letter desegregating all Catholic schools in the St. Louis Archdiocese. While Carter did not graduate from the school, the first Black graduate of SLUH was Al Thomas, also a sophomore transfer and a member of the class of 1950. The first four-year Black graduate was Eldridge Morgan, a member of the class of 1951.

In 1984, Paul Owens became the school's first lay principal. In 2002, Dr. Mary Schenkenberg became its first female principal. In 2005, David J. Laughlin was hired as the school's first lay president.

In each year from 2013 to 2016, SLUH was named among the top-scoring organizations in the mid-size employer category of the St. Louis Post-Dispatch's survey of Top Workplaces.

Since the school is part of the Jesuit network that consists of 61 high schools and 28 colleges and universities in the United States, SLUH provides an education infused with the tradition and philosophy of St. Ignatius of Loyola. Theology and philosophy classes are conducted daily.

According to figures released on SLUH's website in 2011, the median ACT score for SLUH students is over 30. By composite score, it ranks among the top 7% of schools in the United States. Over 50% of SLUH's class of 2011 achieved a score of 30 or higher on the ACT. Among St. Louis and St. Louis area high schools with a total enrollment of over 600, it had the highest scores in 2012. Since 2005 a total of 31 students have received a 36, the highest score possible. Four members of the class of 2012 achieved this score, along with five members of the class of 2013, and two members of the class of 2014. More recently, seven students from the class of 2024 achieved this score.

In 2010, 23 students from SLUH were named National Merit Scholarship Program Semifinalists, more than any other school in Missouri. In 2011, 17 students were named Semifinalists, while 28 were named National Merit Commended Scholars. In 2012, 25 students were named Semifinalists, while 29 were named Commended Scholars.