Peter Isaac Thellusson, 1st Baron Rendlesham (13 October 1761 – 16 September 1808) was a British merchant, banker and politician.
Thelluson was the eldest son of Peter Thellusson, a wealthy London merchant who had emigrated to Britain from France in 1760, and his wife Ann, daughter of Matthew Woodford. After Thellusson senior's death his considerable estate was embroiled in the Thellusson Will Case.
Peter Isaac was educated at Harrow School and took over the thriving family business from his father. Like his father he also became a director of the Bank of England (1787–1806).
He sat as Member of Parliament for Midhurst from 1795 to 1796, for Malmesbury from 1796 to 1802, for Castle Rising from 1802 to 1806 and for Bossiney from 1807 to 1808. In 1806 he was raised to the Peerage of Ireland as Baron Rendlesham, of Rendlesham.
Lord Rendlesham, who lived at Rendlesham Hall, died in September 1808, aged only 46. He had married Elizabeth Eleanor, the daughter of John Cornwall, a Russia merchant of Hendon, Middlesex and was succeeded in the barony by his eldest son John.