University College Dublin (commonly referred to as UCD; Irish: Coláiste na hOllscoile, Baile Átha Cliath) is a public research university in Dublin, Ireland, and a member institution of the National University of Ireland. With 33,284 students, it is Ireland's largest university, and amongst the most prestigious universities in the country. Five Nobel Laureates are among UCD's alumni and current and former staff.

UCD originates in a body founded in 1854, which opened as the Catholic university of Ireland on the Feast of Saint Malachy and with John Henry Newman as its first rector; it re-formed in 1880 and chartered in its own right in 1908. The Universities Act, 1997 renamed the constituent university as the “National University of Ireland, Dublin", and a ministerial order of 1998 renamed the institution as “University College Dublin – National University of Ireland, Dublin".

Originally located at St Stephen's Green in the Dublin city centre, all faculties have since relocated to a 133-hectare (330-acre) campus at Belfield, four kilometres to the south of the city centre. In 1991, it purchased a second site in Blackrock. This currently houses the Michael Smurfit Graduate Business School.

A report published in May 2015 showed the economic output generated by UCD and its students in Ireland amounted to €1.3 billion annually.