The University of the Philippines Diliman (also called UPD; Filipino: Unibersidad ng Pilipinas Diliman), also referred to as UP Diliman, is a public research university located in Diliman, Quezon City, Philippines. It was established on February 12, 1949, to house the central administration of the system of the University of the Philippines System, the national university of the Philippines.

As a member of the University of the Philippines System, it is the fourth oldest and 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.