Isaac Casaubon (/kəˈsɔːbən/; French: [kazobɔ̃]; 18 February 1559 – 1 July 1614) was a classical scholar and philologist, first in France and then later in England.

His son Méric Casaubon was also a classical scholar.

He was born in Geneva to two French Huguenot refugees.^{[citation needed]} The family returned to France after the Edict of Saint-Germain in 1562, and settled at Crest in Dauphiné, where Arnaud Casaubon, Isaac's father, became minister of a Huguenot congregation. Until he was nineteen, Isaac had no education other than that given him by his father. Arnaud was away from home for long periods in the Calvinist camp, and the family regularly fled to the hills to hide from bands of armed Catholics who patrolled the country. It was in a cave in the mountains of Dauphiné, after the St Bartholomew's Day's Massacre, that Isaac received his first lesson in Greek, based on Isocrates' Ad Demonicum.

At the age of nineteen Isaac was sent to the Academy of Geneva, where he read Greek under Franciscus Portus, a Cretan. Portus died in 1581, recommending Casaubon, then only twenty-two, as his successor. He remained at Geneva as professor of Greek until 1596. There he married twice, his second wife being Florence Estienne, daughter of the scholar-printer Henri Estienne. At Geneva, Casaubon lacked example, encouragement and assistance and struggled against the troops of the Catholic dukes of Savoy, but became a consummate Greek and classical scholar. He spent all the money he could spare on books, including copying classics that were not then in print. Even though Henri Estienne, Theodore de Beza (rector of the university and professor of theology), and Jacques Lect (Lectius), were men of superior learning, they often had no time for Casaubon.

Casaubon sought help by cultivating the acquaintance of foreign scholars, as Geneva, the metropolis of Calvinism, received a constant stream of visitors. He eventually met Henry Wotton, a poet and diplomat, who lodged with him and borrowed his money. More importantly, he met Richard Thomson ("Dutch" Thomson), fellow of Clare College, Cambridge, and through Thomson came to the attention of Joseph Scaliger. Scaliger and Casaubon first exchanged letters in 1594. They never met, but kept up a lengthy correspondence that shows their growing admiration, esteem and friendship. Influential French men of letters, the Protestant Jacques Bongars, the Catholic Jacques de Thou, and the Catholic convert Philippe Canaye (sieur de Fresnes) endeavoured to get Casaubon invited to France.

In 1596, they succeeded, and Casaubon accepted a post at the University of Montpellier, with the titles of conseiller du roi (king's advisor) and professeur stipendié aux langues et bonnes lettres (salaried professor of languages and literatures). He stayed there for only three years, with several prolonged absences. He was badly treated and poorly paid by the university authorities. Casaubon began to see the editing of Greek books as a more suitable job for him. At Geneva he had produced some notes on Diogenes Laërtius, Theocritus and the New Testament. He debuted as an editor with a complete edition of Strabo (1587), of which he was so ashamed afterwards that he apologized to Scaliger for it. This was followed by the text of Polyaenus, an editio princeps, 1589; a text of Aristotle, 1590; and a few notes contributed to Estienne's editions of Dionysius of Halicarnassus and Pliny's Epistolae. His edition of Theophrastus's Characteres (1592), is the first example of his peculiar style of illustrative commentary, at once apposite and profuse. When he left for Montpellier he was already engaged upon his magnum opus, his editing of and commentary on Athenaeus.

In 1598 Casaubon was at Lyon, overseeing the printing of his Athenaeus. Here he lived in the house of Méric de Vicq, surintendant de la justice (Superintendent of Justice), a liberal-minded Catholic. Accompanied by de Vicq, Casaubon briefly visited Paris, where he was presented to King Henry IV of France. The king said something about employing Casaubon's services in the "restoration" of the fallen University of Paris. In January 1599, he received a summons to return to Paris, but the terms of the letter were so vague that Casaubon hesitated to act on it. However, he resigned his chair at Montpellier. He stayed another year at Lyon with de Vicq, where he hoped to meet the king, who was expected to visit the south. Nothing more was heard about the professorship, but instead De Vicq summoned him to Paris for important business: the Fontainebleau Conference. Casaubon was persuaded to sit as a referee on the challenge sent to Du Plessis Mornay by Cardinal Duperron. By so doing he placed himself in a false position, as Joseph Scaliger said:

"Non debebat Casaubon interesse colloquio Plessiaeano; erat asinus inter simias, doctus inter imperitos."