Ó Seachnasaigh, O'Shaughnessy, collectively Uí Sheachnasaigh, clan name Cinél nAedha na hEchtghe, is a family surname of Irish origin.

Their name derives from Seachnasach mac Donnchadh, a 10th-century member of the Uí Fiachrach Aidhne, which the Ó Seachnasaigh were the senior clan of.

The town of Gort, Ireland, was the main residence of the family since at least the time of their ancestor, King Guaire Aidne mac Colmáin.

Up until the late 17th century the Ó Seachnasaighs held the sub-district of Uí Fiachrach Aidhne known as Cenél Áeda na hEchtge (modern Irish, Cinéal nAedha na hEchtghe), meaning "kindred of Aedh of the Slieve Aughty", which was also their clan name.

The last de facto Ó Seachnasaigh lord of Kinelea died at Gort after returning home from the Battle of Aughrim on 12 July 1691.