County Westmeath (/wɛstˈmiːð/; Irish: Contae na hIarmhí or simply An Iarmhí) is a county in Ireland. It is in the province of Leinster and is part of the Eastern and Midland Region. It formed part of the historic Kingdom of Meath, which was named Mide because the kingdom was located in the geographical centre of Ireland (the word Mide meaning 'middle'). Westmeath County Council is the administrative body for the county, and the county town is Mullingar. At the 2022 census, the population of the county was 95,840.

Following the Norman invasion of Ireland, the territory of the Gaelic Kingdom of Meath formed the basis for the Anglo-Norman Lordship of Meath granted by King Henry II of England to Hugh de Lacy in 1172. Following the failure of de Lacy's male heirs in 1241, the Lordship was split between two great-granddaughters. One moiety, a central eastern portion, was awarded to Maud (de Geneville) as the liberty of Trim; the other moiety, comprising north-eastern and western portions, went to Maud's sister Margery (de Verdun) and in 1297 became the royal county of Meath. The liberty and royal county were merged in 1461. While the east of the county was in the English Pale, the west was Gaelicised in the fourteenth century and outside the control of the sheriff of Meath.

As a part of the better administration of the newly established Kingdom of Ireland, the Parliament of Ireland passed the Counties of Meath and Westmeath Act 1543, the eastern portion retaining the name Meath and the western portion called Westmeath.

Westmeath is the 20th largest of Ireland's 32 counties by area and the 22nd largest in terms of population. It is the sixth largest of Leinster's 12 counties in size and eighth largest in terms of population. The Hill of Uisneach in the barony of Moycashel is sometimes regarded as the notional geographical centre of Ireland although the actual geographic centre of Ireland lies in neighbouring County Roscommon. The summit of Mullaghmeen is the highest point in County Westmeath. At just 258 metres this makes it the lowest county top in Ireland.

The head office of Westmeath County Council is located in Mullingar. There are currently 20 councillors. The three local electoral areas of Westmeath are Athlone (7 seats), Mullingar–Coole (7 seats) and Mullingar–Kilbeggan (6 seats). The Local Government (Ireland) Act 1898 provided the framework for the establishment of County Councils throughout Ireland. The first meeting of Westmeath County Council was held on 22 April 1899.

Westmeath's population growth has been stronger than the national average. After the Great Famine, the population of Westmeath declined dramatically. It stabilised in the middle of the 20th century, and has continued to grow. Westmeath's proximity to Dublin, with good motorway facilities and frequent rail service, has made commuting popular.

County Westmeath's population fell in the century following the Great Famine, with many leaving for better opportunities in America. The largest town in the county is Athlone, followed by Mullingar. Westmeath is the largest county by population in the Irish Midlands. Important commercial and marketing centres include Moate, Kilbeggan, Kinnegad, Ballinahown, Delvin, Rochfortbridge, Killucan and Castlepollard. According to the 2011 census, 51.9% of Westmeath households have at least one member reporting an ability in Irish.

Westmeath is one of the few counties in Ireland where some census records from 1841 are still available. Some of the records of that census have been digitised and maintained by the National Archives of Ireland.