county in the province of Ulster, northeastern Ireland. It is bounded by Counties Monaghan (northeast), Meath, Westmeath, and Longford (south), and Leitrim (northwest). Northern Ireland lies to the north. Northwestern Cavan comprises uplands, intersected by valleys, declining toward the main valley of the River Erne, where the main features are the drumlins (long, oval mounds) that alternate with lakes, rivers, peat bogs, and pastures. East of the Erne Valley is a line of slate and shale hills.

The Cavan district is mentioned in accounts of the life of St. Patrick (Patrick, Saint) (flourished 5th century). Cavan formed part of the kingdom of Bréifne, which included Leitrim, of which the O'Rourkes were princes. In later times Cavan, or East Bréifne, became distinctively Bréifne O'Reilly; and West Bréifne, or Leitrim, became Bréifne O'Rourke. Bréifne long resisted colonization by the Anglo-Normans, and the O'Reilly's of Cavan were not brought under permanent English rule until the late 16th century. Cavan, previously part of Connacht, was designated a part of Ulster in the early 17th century and included in the Ulster plantation from 1608 onward, when it was settled by Scots and English colonists.

County Cavan is predominantly rural. Cavan, the county seat, Cootehill, and Belturbet are the largest towns in the county. The Roman Catholic cathedral of Kilmore diocese is in Cavan, the Protestant cathedral in Kilmore. Agriculture dominates the county economy. Creameries are common, and the towns are market centres with some light industry. Main roads converge on Cavan from Belfast, Dublin, and Athlone. Area 746 square miles (1,932 square km). Pop. (2002) 56,546.