Letterkenny ("hillside of the O'Cannons", nicknamed "the Cathedral Town", is the largest and most populous town in County Donegal, Ireland. It lies on the River Swilly in East Donegal and has a population of 19,274. It is the 36th largest settlement in all of Ireland by population (placing it ahead of Sligo, Armagh and Killarney) and is the 15th largest settlement by population in the historic province of Ulster (most of which comprises the separate jurisdiction of modern-day Northern Ireland). Along with the nearby city of Derry, Letterkenny is considered a regional economic gateway for the north-west of Ireland. Letterkenny acts as an urban gateway to the Ulster Gaeltacht, similar to Galway's relationship to the Connemara Gaeltacht.

Letterkenny began as a market town at the start of the 17th century, during the Plantation of Ulster. A castle once stood near where the Cathedral of St Eunan and St Columba, County Donegal's only Catholic cathedral, stands today. Letterkenny Castle, built in 1625, was located south of Mt Southwell on Castle Street. County Donegal's premier third-level institution, the Letterkenny Institute of Technology (LYIT), is located in the town, as are St Eunan's College, Highland Radio, and a Hindu temple. Letterkenny was also the original home of Oatfield Sweet Factory, a confectionery manufacturer; the factory closed and was demolished in 2014. In 1798, Theobald Wolfe Tone was arrested at Laird's Hotel in the town. In 2015, Letterkenny was judged as the tidiest town in Ireland.