Castlefin (Irish: Caisleán na Finne, meaning 'castle of the (river) Finn'), sometimes spelt Castlefinn, is a market town and townland in the Finn Valley of County Donegal, Ireland. It is located between Stranorlar and Lifford in East Donegal. As of 2022, the population was 730. The River Finn flows by the town. The town is located in along the main N15 national primary road, which runs from Bundoran to Lifford. The town lies 6 miles from Lifford and 8 miles from the twin towns of Ballybofey and Stranorlar. It has close links to Letterkenny, to the twin towns of Ballybofey and Stranorlar, and to West Tyrone in Northern Ireland, especially with the towns of Strabane and Castlederg.

The five housing estates in the town are called the Emmett Park built in the 1980s Sessaigh Park built in the 2000s, Caislean Court built in the 1990s, Hillhead built in the 1970s and Grahamsland built in the 1950s.

The town centre is located around the Diamond area, which is where three routes merge. The Diamond has landscaped seating and planting areas, and the surrounding area also has a number of buildings that are included on the Record of Protected Structures. The town has a number of retail, commercial, religious, economic, social and recreational amenities. Castlefinn also serves as a focus of primary education for the surrounding rural areas. The local national school has a large catchment area and currently has 180+ pupils attending. The town also has a pre-school.

The town centre has a number of amenities including retail outlets, grocery stores, a petrol station, post office, butchers, take-aways, public houses and some hairdressers. The town also has a recycling facility that is located on the Castlederg road beside the bridge.

The C.P.I center is used for football, parties, computing and the youth club. There is a local GAA club (Robert Emmetts) and a soccer field (Castlefinn Celtic). Castlefinn is also the home of Finn Valley Radio which broadcasts on 95.8FM locally and online. The station holds a community licence and its studios are located at the CPI Centre.

Local schools include St Mary's National School, Scoil Náisiúnta Domhnach Mór (Liscooley) and St. Safan's National School (Dunnyloop).

Castlefinn railway station opened on 7 September 1863, but finally closed on 1 January 1960.

A number of buses pass through Castlefinn on a daily basis going to Letterkenny, Derry, Strabane, Dublin, Sligo and Galway.

Castlefin is in the Donegal constituency for Dáil elections. There are currently five TDs for this constituency, Pearse Doherty and Padraig MacLochlainn (Sinn Féin), Charlie McConalogue (Fianna Fáil), Thomas Pringle (Independent) and Joe McHugh, (Fine Gael).

The town is in the Lifford-Stranorlar Municipal District for local elections to Donegal County Council. The six Councillors elected to represent the area are Gary Doherty and Liam Doherty (Sinn Féin), Patrick McGowan and Gerry Crawford (Fianna Fáil), Martin Harley (Fine Gael) and Frank McBrearty (Independent).

• Seán Reid, Musician

• List of populated places in the Republic of Ireland

• Donegal County Council - Castlefinn Local Area Plan