| Viscountcy Southwell | |
| Creation Date: | 18 July 1776 |
| Present Holder: | Richard Andrew Pyers Southwell, 8th Viscount |
| Heir Apparent: | The Hon. Charles Anthony John Southwell |
| Remainder To: | Heirs male of the first viscount's body, lawfully begotten |
| Subsidiary Titles: | Baron Southwell Baronet |
| Former Seat: | Nec Male Notus Eques (Not an unknown knight) |
| Motto: | Deo Favente |
Viscount Southwell, of Castle Mattress in the County of Limerick, is a title in the Peerage of Ireland. It was created in 1776 for Thomas Southwell, 3rd Baron Southwell. The Southwell family descends from Thomas Southwell. In 1662 he was created a Baronet, of Castle Mattress in the County of Limerick, in the Baronetage of Ireland. He was succeeded by his son, the second Baronet. He represented County Limerick in the Irish Parliament. In 1717 he was created Baron Southwell, of Castle Mattress, in the County of Limerick, in the Peerage of Ireland. His grandson was the aforementioned third Baron, who was elevated to a viscountcy in 1776. Before succeeding in the barony he had represented Enniscorthy in the Irish House of Commons. His great-grandson, the fourth Viscount, served as Lord Lieutenant of County Leitrim between 1872 and 1878. the titles are held by his great-great-grandson, the eighth Viscount, who succeeded his father in that year.
The heir presumptive and last in line to the titles is the present holder's younger brother, the Hon. Charles Anthony John Southwell (born 1962).
(ancestors of 7th Viscount)