Dún Laoghaire (Irish pronunciation: [d̪ˠuːn̪ˠ ˈɫeːrʲə]), sometimes unofficially spelt using modern Irish orthography as Dún Laoire, is a suburban seaside town in County Dublin, Ireland. It is about twelve kilometres south of Dublin city centre. Dún Laoghaire is the county town of Dún Laoghaire–Rathdown county and formerly a major port of entry from Great Britain. From 1821 to 1921 it was officially called "Kingstown".

The town's name derives from Lóegaire mac Néill, a 5th century High King of Ireland, who chose the site as a sea base from which to carry out raids on Britain and Gaul. Dún is an Irish word meaning fort, and traces of fortifications from that period have been found on the coast. Some stone from that fort is preserved in the Maritime Museum.

In rare cases, the anglicized spelling "Dunleary" (English pronunciation: /dʌnˈlɪəri/) is used.

The present town of Dún Laoghaire dates from the 1820s. An earlier Dún Laoghaire village was located around the area where "The Purty Kitchen" pub is now located (sometimes mapped as "Old Dún Laoghaire"). Dún Laoghaire had a coffee house, and a small cove, both of which are shown on a number of old maps, and it may have had a salt mine (Salthill is close by). At that time, the area on which the town is currently located was a craggy, rocky pasture area with some quarries.