Sir John Spencer (died 1610) was a successful English merchant and lord mayor of London.
He was the son of Richard Spencer of Waldingfield in Suffolk, came to London, and as a merchant was nicknamed "Rich Spencer". His trade with Spain, Turkey, and Venice was substantial, and he was accused in 1591 of engrossing, with two other merchants, the whole trade with Tripoli. Queen Elizabeth I is said to have visited him at Canonbury House in 1581, a property he bought from Thomas Wentworth, 2nd Baron Wentworth in 1570.
Spencer was a member of the Clothworkers' Company, and was elected alderman of Langbourn ward on 9 August 1587. He served the office of sheriff of London in 1583–4, and that of lord mayor in 1594–5. During his shrievalty he was engaged in hunting down papists in and around Holborn and the adjoining localities, and had to justify before the council the committal of Anthony Bassano and others among her majesty's musicians. The end of 1594 was a time great scarcity prevailed, and Spencer sent his precept to the city companies to replenish their store of corn at the granaries in the Bridge House for sale to the poor. He resisted a demand by Admiral Sir John Hawkins for possession of the Bridge House for the use of the navy and baking biscuits for the fleet.
Spencer kept his mayoralty at his town residence, Crosby Place in Bishopsgate Street, which he had purchased and restored (it had passed from Antonio Bonvisi to alderman William Bond, father of Martin Bond). In this sumptuous mansion during the course of 1604 Spencer entertained both the Duc de Sully (then M. de Rosny), while ambassador to England, and the youngest son of the Prince of Orange, with Barnevelt and Fulke, who came on a mission from Holland. Towards the close of his mayoralty he asserted the City of London's right, which it was feared the Crown would contest, to freely elect a recorder. Before the close of his mayoralty he received a knighthood.