John Keigwin (1641–1716) was a Cornish antiquary, born at Mousehole, Cornwall. He was a leading member of a group of antiquaries in west Penwith; this group also included John Boson and Thomas Boson, William Gwavas, Thomas Tonkin, William Borlase, Oliver Pender, and James Jenkins of Alverton. The scholars Edward Lhuyd and William Borlase described Keigwin's knowledge of the Cornish language as profound and complete. His teacher was the scholar John Boson. Keigwin had mastered the French, Latin, Greek and Hebrew languages in addition to Cornish and English. He was a nephew of William Scawen, also a scholar of the Cornish language, since Scawen's sister Elizabeth had married his father Martin Keigwin.
Keigwin's translations of Pascon Agan Arluth and Creacon of the World were published by Davies Gilbert in 1826 and 1827.
Among other scholarly work he translated the letter of Charles I, King of England to the people of Cornwall written at Sudeley Castle in 1643 into Cornish. The scholar Henry Jenner, noted extraordinary mistakes made by Keigwin in the translations of Pascon and Creacon. Jenner and P. Berresford Ellis note that Keigwin used the Hebrew word for war, milchamath, in translating King Charles's letter instead of bresel.
R. Morton Nance regarded Keigwin's Cornish as less good but E. G. Retallack Hooper maintained that criticism of his Cornish is partly due to the way Davies Gilbert edited his writings. As Gilbert did not understand any Cornish himself and could not read Keigwin's handwriting properly, though Keigwin's reputation in Cornwall was good his work was neglected until it was reexamined by Whitley Stokes and others.