Sir Théodore Turquet de Mayerne (28 September 1573 – 22 March 1655) was a Genevan-born physician who treated kings of France and England and advanced the theories of Paracelsus.

Mayerne was born in a Huguenot family in Geneva, Republic of Geneva. His father was a Protestant French historian^{[citation needed]} who had fled Lyon following the St. Bartholomew's Day Massacre and his godfather was Theodore Beza. Mayerne's first wife was Marguerite de Boetslaer and they had three children.

Mayerne studied first in Geneva and later moved to the University of Heidelberg. Later he moved to Montpellier to study medicine, graduated 1596 and received his doctorate in 1597. His dissertation defended the use of chemical remedies in medicine, under the guidance of Joseph du Chesne; this was the first intimation of his interest in Paracelsian theories. In May 1599, Mayerne joined Henri de Rohan, a Huguenot nobleman very powerful in Brittany, on his grand tour of Europe, visiting Germany, Italy, Bohemia, the Netherlands, England, and Scotland. During their visit to London in October 1600, Rohan and Mayerne were received by Queen Elizabeth I at her court, and in November 1600, when they reached Edinburgh, they were received by King James VI, before returning to France in early 1601. Despite his austere Calvinism, Mayerne greatly admired the many works of art and architecture he saw in his travels in Germany and Italy, especially liking the paintings of Albrecht Dürer and the Kunstkammer in Munich of curios kept by the Duke of Bavaria.

Mayerne moved to Paris, lectured on anatomy and pharmacy and founded a medical practice. By that time he had begun to support the views of Paracelsus and used many chemical remedies. He kept detailed notes about his patients, among them Armand du Plessis, later Cardinal Richelieu, whom he treated for gonorrhea in 1605.

In 1600, French royal physician [d] Jean Ribit de la Rivière [fr] (1571–1605) sponsored him to become one of the personal physicians, physician in ordinary, of the king Henri IV. His other pursuits were thwarted because he was not a Catholic and because most French physicians still followed the principles of Galen. In 1603 he tried to support his views to Medical Faculty of Paris, stating that his views were not opposed to Galenic and Hippocratic principles.

Despite their opposition, he retained the favour of the King, who appointed him to travel with the Duc de Rohan in his diplomatic missions to Germany and Italy. When the King intended to make Mayerne his first physician, the queen opposed the decision because Mayerne refused to convert to Catholicism. Mayerne continued in his lower post until 1606 when he sold it to another physician.

At this time he continued his association with du Chesne and the circle of Hermeticists that had grown up around him. These devotees of Paracelsus believed they were reviving the wisdom of the mythical pre-Platonic natural philosophers - men known as the prisci theologi that included Zoroaster and Hermes Trismegistus. The alchemical nature of their experiments was greatly resented by Galenists at the University of Paris.

In the same year he briefly visited England by invitation and met James I. He became a physician of Anne of Denmark and was incorporated at Oxford on April 8, 1606. He probably spent the following years back in France.