Sir Thomas Southwell, 1st Baronet (died 7 December 1680) was an Anglo-Irish politician.
Southwell was the son of Edmund Southwell and Catherine Herbert. His grandfather, John Southwell, had come to Ireland from Suffolk in the reign of James I.
He served as High Sheriff of Kerry, High Sheriff of Clare and High Sheriff of County Limerick in 1654 under The Protectorate. On 4 August 1662 he was created a baronet, of Castle Mattress in the Baronetage of Ireland, by Charles II.
He married Elizabeth Starkey, daughter of William Starkey. His eldest son, the politician Richard Southwell, predeceased him and he was succeeded in his title by his grandson, Thomas Southwell, 1st Baron Southwell.