Castlederg (earlier Caslanadergy, from Irish Caisleán na Deirge, meaning 'castle on the Derg') is a town in County Tyrone, Northern Ireland. It lies on the River Derg and is near the border with County Donegal, Ireland. It stands in the townlands of Castlesessagh and Churchtown, in the historic barony of Omagh West and the civil parish of Urney. The village has a ruined castle and two ancient tombs known as the Druid's Altar and Todd's Den. It had a population of 2,980 people at the 2021 Census.
 The village hosts some of the district's key events each year, including the Derg Vintage Rally, Dergfest music festival, Red River Festival and the traditional Apple Fair.
 Castlederg was a traveller's stop along the ancient pilgrimage route to Station Island on Lough Derg. The town boasts ancient ruins and monastic settlements.
 Historically the area around the town was a site of contestation between the territories of Cenél nEógain (later Tír Eoghain) and Connail (later Tír Chonaill - mostly modern County Donegal). This rivalry between the two powers continued until the 16th century when they combined in the defence of Ulster against the encroaching Elizabethan armies.  The Castlederg area, lying within the new barony of Omagh, was granted to the English Attorney-General for Ireland, Sir John Davies. Two castles were constructed on his proportion, Castle Curlews (Kirlish Castle) outside Drumquin and Derg Castle, the ruins of which can be seen today on the northern bank of the River Derg at Castlederg. A bronze-age cauldron was found at Castlederg in 2011.
 During the Troubles, 25 people were killed in and around Castlederg (including Killeter and Killen) and there were many bombings in the village. The Provisional Irish Republican Army (IRA) killed 11 members of the Ulster Defence Regiment and Royal Ulster Constabulary, four fellow IRA members whom it accused of being informers, and three Ulster Protestant civilians. Four IRA members were also killed when their bombs exploded prematurely. Ulster loyalist paramilitaries killed three Catholic civilians. For more information, see The Troubles in Castlederg.
 The narrow-gauge Castlederg and Victoria Bridge Tramway was built in 1883, to link the village with the Great Northern Railway (Ireland) at Victoria Bridge. Castlederg railway station opened on 4 July 1884, but was finally closed on 17 April 1933.
 The population of the village increased during the 19th century:
 Castlederg is classified as an intermediate settlement by the Northern Ireland Statistics and Research Agency (NISRA) (i.e. with a population between 2,500 and 4,999 people).
On Census Day (27 March 2011) the usually resident population of Castlederg Settlement was 2,976, accounting for 0.16% of the NI total. Of these:
 On Census Day (21 March 2021) the usually resident population of Castlederg Settlement was 2,980. Of these:
 Castlederg recorded Northern Ireland's lowest-ever recorded temperature of −18.7 °C (−1.7 °F) on the morning of 23 December 2010. The town recorded Northern Ireland's highest-ever recorded temperature of 31.3 °C (88.3 °F) on 21 July 2021. On the 22 July, Armagh reported 31.4 °C (88.5 °F) which has since been rejected by the UK Met Office, meaning Castlederg holds both the highest and lowest temperature records in Northern Ireland. Prior to the 21st and 22nd, the record was also broken on 17 July 2021 with a value of 31.2 °C (88.2 °F) at Ballywatticock.
 The town is one of the electoral wards in the Derg district electoral area of Derry City and Strabane District Council. The other wards are Finn, Glenderg, Newtownstewart and Sion Mills. Below are the results of the 2019 Derry City and Strabane District Council election
 2014: 3 x Sinn Féin, 1 x DUP, 1 x UUP
2019: 2 x Sinn Féin, 1 x DUP, 1 x UUP, 1 x SDLP
2014-2019 change: SDLP gain one seat from Sinn Féin