The University of the Philippines Diliman (UP Diliman or UPD; Filipino: Unibersidad ng Pilipinas Diliman) is a public research university and the lead constituent university of University of the Philippines System, located in Diliman, Quezon City, Philippines. Established in 1949, UP Diliman originated from the transfer of most of the university’s academic functions from Manila, where its founding colleges had operated since 1909, as part of the planned expansion of the national university. It was developed as the main campus of the University of the Philippines during the period when the university functioned as a unified institution.

UP Diliman is the largest constituent campus in terms of the number of degree-granting academic units, student population, faculty, and library resources. There are 27 degree-granting units on campus, accounting for 26,349 students of which, 17,117 are undergraduates. UP Diliman had a complement of 1,620 regular faculty in 2023, of whom 499 have doctoral degrees.

In addition to the units in the main campus, UP Diliman has extension programs in Angeles City, Pampanga (the Clark Freeport Zone area) and Olongapo, Zambales, as well as a marine laboratory in Bolinao, Pangasinan under the Marine Science Institute, and a satellite campus at Bonifacio Global City, Taguig. The UP Diliman campus is also the site of the country's National Science Complex. UP Diliman offers academic programs in 247 major fields. There are 70 programs at the undergraduate level, 109 programs at the master's level and 68 at the doctoral level.