Leinster ( LEN-stər; Irish: Laighin [ˈl̪ˠəinʲ] or Cúige Laighean [ˌkuːɟə ˈl̪ˠəinˠ]) is one of the four provinces of Ireland, situated in the southeast and east of Ireland. The province comprises the ancient Kingdoms of Meath, Leinster and Osraige. Following the 12th-century Norman invasion of Ireland, the historic "fifths" of Leinster and Meath gradually merged, mainly due to the impact of the Pale, which straddled both, thereby forming the present-day province of Leinster. The ancient kingdoms were shired into a number of counties for administrative and judicial purposes. In later centuries, local government legislation has prompted further sub-division of the historic counties.

Leinster has no official function for local-government purposes. However, it is an officially recognised subdivision of Ireland and is listed on ISO 3166-2 as one of the four provinces of Ireland. "IE-L" is attributed to Leinster as its country sub-division code. Leinster had a population of 2,858,501 according to the preliminary results of the 2022 census, making it the most populous province in the country. The traditional flag of Leinster features a golden harp on a green background.