John William Ponsonby, 4th Earl of Bessborough, PC (31 August 1781 – 16 May 1847), known as Viscount Duncannon from 1793 to 1844, was a British Whig politician.

Ponsonby's mother was Lord Granville's lover prior to his marriage to Lady Harriet Cavendish, the Countess of Bessborough's niece.

He was made a Privy Counsellor in 1831 and in 1834, ten years before he succeeded his father, he was created Baron Duncannon, of Bessborough in the County of Kilkenny.

In private on the other hand, he was regarded as a valued colleague in Government, due largely to his ability to keep his head in a crisis.

[2][3] Their daughter Lady Emily Charlotte Mary remained unmarried but she wrote a number of novels which were published without attribution.