Viscount Southwell (/ˈsʌðəl/ SUDH-əl), of Castle Mattress in the County of Limerick, is a title in the Peerage of Ireland.

In 1662 he was created a Baronet, of Castle Mattress in the County of Limerick, in the Baronetage of Ireland.

In 1717 he was created Baron Southwell, of Castle Mattress, in the County of Limerick, in the Peerage of Ireland.

As of 2019[update] 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.