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.

He was educated at Eton College and Corpus Christi College, Cambridge and studied law at the Middle Temple, where he was called to the bar in 1742.^[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. In 1754 his mother inherited Belton House from her childless brother, John Brownlow, 1st Viscount Tyrconnel, the last male of the Brownlow family. He was unanimously elected Speaker of the House of Commons in 1761,^[2] and unanimously reelected in 1768.^[3] He was admitted to the Privy Council in 1762.^[4]

On 17 January 1770, Cust sent word to the House of Commons that he was too ill to attend.^[5] He resigned on 19 January; his successor Sir Fletcher Norton was elected on 22 January. Cust died on 24 January 1770, aged 51.^[4] His memorial in Belton church was created by William Tyler RA.^[6]

In 1743 he married Etheldreda Payne, a daughter of Thomas Payne, by whom he had two sons and two daughters, including:

• Brownlow Cust, 1st Baron Brownlow, 4th Baronet (1744–1807), who succeeded his father in the baronetcy and in 1776 was raised to the peerage as Baron Brownlow in recognition of his father's services.
• Elizabeth Cust (1750–1779), married Philip Yorke I (1743–1804) of Erddig House, as his first wife.