Sir John Cust, 3rd Baronet PC (29 August 1718 – 24 January 1770), of Belton House near Grantham in Lincolnshire, was a British politician who served as Speaker of the House of Commons from 1761 to 1770.

He was the eldest son of Sir Richard Cust, 2nd Baronet (1680–1734) by his wife Anne Brownlow, daughter of Sir William Brownlow, 4th Baronet, of Belton House, and heiress in her issue[clarification needed] of her brother John Brownlow, 1st Viscount Tyrconnel, 5th Baronet (1690–1754), of Belton House.

[1] He was elected as a Member of Parliament for Grantham in 1743, which seat he continued to represent until his death 27 years later.

[4] On 17 January 1770, Cust sent word to the House of Commons that he was too ill to attend.

[4] His memorial in Belton church was created by William Tyler RA.