County Mayo (/ˈmeɪoʊ/; from Irish Maigh Eo, Contae Mhaigh Eo, meaning 'Plain of the yew trees') is a county in Ireland. In the West of Ireland, in the province of Connacht, it is named after the village of Mayo, now generally known as Mayo Abbey. Mayo County Council is the local authority. The population was 137,231 at the 2022 census. The boundaries of the county, which was formed in 1585, reflect the Mac William Íochtar lordship at that time.

It is bounded on the north and west by the Atlantic Ocean; on the south by County Galway; on the east by County Roscommon; and on the northeast by County Sligo. Mayo is the third-largest of Ireland's 32 counties in area and 18th largest in terms of population. It is the second-largest of Connacht's five counties in both size and population. Mayo has 1,168 km (726 mi) of coastline, or approximately 21% of the total coastline of the State. It is one of three counties which claims to have the longest coastline in Ireland, alongside Cork and Donegal. There is a distinct geological difference between the west and the east of the county. The west consists largely of poor subsoils and is covered with large areas of extensive Atlantic blanket bog, whereas the east is largely a limestone landscape. Agricultural land is therefore more productive in the east than in the west. Clew Bay lies on the west coast of the county.

There are nine historic baronies, four in the northern area and five in the south of the county:

North Mayo

South Mayo

According to the 2022 census:

A survey of the terrestrial and freshwater algae of Clare Island was made between 1990 and 2005 and published in 2007. A record of Gunnera tinctoria is also noted.

Consultants working for the Corrib gas project have carried out extensive surveys of wildlife flora and fauna in Kilcommon Parish, Erris between 2002 and 2009. This information is published in the Corrib Gas Proposal Environmental impact statements 2009 and 2010.