County Offaly (Irish: Contae Uíbh Fhailí) is a county in Leinster, Ireland, bordered by seven other counties: Galway, Roscommon, Westmeath, Meath, Kildare, Laois, and Tipperary. Originally the Kingdom of Uí Failghe, it was shired in 1556 by Mary I of England during one of the Plantations of Ireland, it was originally named King's County, after King Philip II of Spain and the county town was Daingean until the nineteenth century.

The southern part of the county is occupied by the Slieve Bloom Mountains, the northwestern part is the floodplain of the River Shannon and the remainder consists of large bogs including the Bog of Allen and Boora Bog.

Birr in the south of the county is best known for its castle and gardens. Birr Castle is owned by the Parsons family, (the family bears the title: 'Earl of Rosse') and is best known for its 19th century telescope.

Before the plantation of modern day Offaly by English settlers, the territories it comprises were controlled principally by the Ó Conchubhair Fáilghe clan in the North/East of the county and the Ó Céarbhaill of Ely in the south.

The short story writer, novellist and playwright William Trevor (KBE) was born in County Offaly, and raised in County Cork, Irish Free State, and has written of his experiences growing up as a Protestant in that time and place.