Francisco Sanches (/ˈsɑːntʃɛs/; Spanish: [ˈsantʃes]; c. 1550 – November 16, 1623) was a skeptic, philosopher and physician of Sephardi Jewish origin, born possibly in Tui, Spain or probably in Braga, Portugal (where he was baptized and grew up).

Although the investigations carried out by Henry Pierre Cazac at the beginning of the 20th century – he presented, among other documents, an autograph by Sánchez that reads as follows: "Ego, Franciscus Sanctius, Hispanus, natus in civitate Tudensi [...]" – showed the Spanish origin of Francisco Sánchez, there has been a long controversy around his nationality, as shown in the title of some publications that try to ascribe the name of the illustrious doctor and philosopher to the French Renaissance (The Skeptics of the French Renaissance, by John Owen) or Portuguese (Francisco Sanches e a dúvida metódica na Renascença portuguesa, doctoral thesis by Evaristo de Moraes Filho).

[9] His father was the Spaniard Antonio Sanches, also a physician; his mother Filipa de Sousa was Portuguese.

He studied in Braga until the age of 12, when he moved to Bordeaux with his parents, fleeing the surveillance of the Portuguese Inquisition.

This early formulation of "constructive" or "mitigated" skepticism was to be developed into an important explication of the new science by Marin Mersenne, Pierre Gassendi, and the leaders of the Royal Society.