County Donegal is a county in the northwest of Ireland. It is one of three counties in the province of Ulster that does not form part of Northern Ireland. The county was named after the former administrative centre of Donegal Town. When first created, it was sometimes referred to as County Tyrconnel (Irish: Tír Chonaill), after the Tyrconnel earldom it succeeded. Calling the whole county Tír Conaill is technically incorrect as the Inishowen peninsula (Irish: Inis Eoghain) was historically distinct from Tír Chonaill.

Uniquely, Donegal only shares a border with one county in the Republic of Ireland, named County Leitrim in north Connacht. The rest of its land border is shared with the Northern Irish counties of Derry, Tyrone, and Fermanagh. This apparent isolation has led to Donegal people and their customs being considered distinct from the rest of the country and has been used to market the county with the slogan Up here it's different. Despite Lifford being the county town (and there also being a Donegal town), the largest town is Letterkenny.

The county consists chiefly of low mountains, with a deeply indented coastline forming natural loughs, of which Lough Swilly is the most notable. The famous mountains or Hills of Donegal consist of two major ranges, the Derryveagh Mountains in the north and the Bluestack Mountains in the south, with Mount Errigal at 751 metres the highest peak. The Slieve League cliffs are the second highest sea cliffs in Europe, while Donegal's Malin Head is the most northernly point on the island of Ireland.

The climate is temperate and dominated by the Gulf Stream, with cool damp summers and mild wet winters. Two permanently inhabited islands, Arranmore and Tory Island, lie off the coast along with a large number of islands with only transient inhabitants. Ireland's second longest river, the Erne, enters Donegal Bay near the town of Ballyshannon. The river Erne, along with other Donegal waterways, has been dammed to produce hydroelectric power.

An extensive rail network used to exist through out the county and was operated by the County Donegal Railways Joint Committee and the Londonderry and Lough Swilly Railway Company. Even though the railways in Donegal are fondly remembered, the network was completely closed by 1960. The county is served by Donegal Airport.