THIS MASTERPIECE BY ENGINEER-FRIAR Fr. Roque Ruaño, O.P., who once served as Dean of the Faculty of Engineering, was the first structure on the Sampaloc campus. From its inauguration in 1927 until 1963, the Main Building was occupied by the following: the Fathers as their residence and the home of the Ecclesiastical Faculties (1923-1927), Faculty of Medicine (1927-1950), College of Philosophy and Letters (1927-1967), Faculty of Engineering (1927-1950), College of Education (1927-1950), Central Library (1927-1992), College of Commerce (1930-1967), UST High School (1930-1946), and the radio station DZST (1950-1963), which later became Radio Veritas.

At present, the Main Building houses the administrative offices, some non-academic departments, the UST Museum of Arts and Sciences and three colleges: the Faculty of Pharmacy, College of Science, and the Faculty of Civil Law.

During the Second World War, the Main Building was used by the Japanese military forces as an internment camp for allied prisoners. The Main Building holds the distinction of being the first earthquake-resistant building in the Philippines, a Thomasian engineer’s contribution to construction technology in the Philippines.