County Longford (Irish: Contae an Longfoirt) is a county in Ireland. It is in the province of Leinster. It is named after the town of Longford. Longford County Council is the local authority for the county. The population of the county was 46,634 at the 2022 census. The county is based on the historic Gaelic territory of Annaly (Anghaile), formerly known as Teffia (Teathbha).

Most of Longford lies in the basin of the River Shannon with Lough Ree forming much of the county's western boundary. The north-eastern part of the county, however, drains towards the River Erne and Lough Gowna. Lakeland, bogland, pasture-land and wetland typify Longford's generally low-lying landscapes: the highest point of the county is in the north-west – Carn Clonhugh (also known as Cairn Hill or Corn Hill) between Drumlish and Ballinalee in the parish of Killoe, at 278 metres (912 ft). Cairn Hill is the site of a television transmitter broadcasting to much of the Irish midlands.

In the list of Irish counties by highest point, Longford ranks third lowest – only Meath and Westmeath have lower maxima. In general, the northern third of the county is hilly, forming part of the drumlin belt and Esker Riada stretching across the northern midlands of Ireland. The southern parts of the county are low-lying, with extensive areas of raised bogland and the land being of better quality for grazing and tillage. The River Shannon marks the county's border with Roscommon while the Rivers Inny and Tang form much of the boundary with Westmeath.

The Royal Canal flows through the south of the county, terminating at Cloondara at the Shannon. The canal was refurbished and reopened in 2010, 39 years after its closure by the CIÉ. Notable lakes include Lough Kinale and Lough Gowna on the County Cavan border, Lough Forbes on the County Roscommon border and Lough Ree in the south where counties Longford, Westmeath and Roscommon meet.

There are six historical baronies:

Townlands are the smallest officially defined geographical divisions in Ireland, with 944 townlands in the county.

The county is part of the Dáil constituency of Longford–Westmeath.

The territory corresponding to County Longford was presumably a frontier colony of the Kingdom of Meath in the first millennium. Between the fifth and twelfth centuries, the territory was called the kingdom of Tethbae ruled by various tuath such as the Cairpre Gabra in the north. Tethbae (Latin: Teffia) originally referred to an area north of the River Inny approximating to present-day County Longford.