Sir John Spencer (died 1610) was a successful English merchant and Lord Mayor of London.
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 of great scarcity, 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.
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.
He was buried on 22 March, and Dame Alice on 7 April, in his parish church of St Helen, Bishopsgate, where an altar tomb monument was placed to his memory.
His fortune was variously estimated at very large sums, and the splendid inheritance is said for the time to have turned the brain of his son-in-law, Lord Compton.