John Greaves (1602 - October 8, 1652), English mathematician and antiquary, was the eldest son of John Greaves, rector of Colemore, near Alresford in Hampshire.

He was educated at Balliol College, Oxford, and in 1630 was chosen professor of geometry in Gresham College, London . After travelling in Europe, he visited the East in 1637, where he collected a considerable number of Arabic, Persian and Greek manuscripts, and made a more accurate survey of the pyramids of Egypt than any traveller who had preceded him.

On his return to Europe he visited a second time several parts of Italy, and during his stay at Rome instituted inquiries into the ancient weights and measures. In 1643 he was appointed to the Savilian professorship of astronomy at Oxford, but he was deprived of his Gresham professorship for having neglected its duties. In 1645 he essayed a reformation of the calendar, but his plan was not adopted. In 1648 he lost both his fellowship and his Savilian chair on account of his adherence to the royalist party. But his private fortune more than sufficed for all his wants till his death.

Besides his papers in the Philosophical Transactions, the principal works of Greaves are:

• Pyramidographia, or a Description of the Pyramids in Egypt (1646)
• A Discourse on the Roman Foot and Lienarius (1649)
• Elementa Linguae Persicae (1649)

His miscellaneous works were published in 1737 by Dr Thomas Birch, with a biographical notice of the author. See also Smith's Vita quorundam erudit. virorum and John Ward's Gresham Professors.