Johan de Witt (24 September 1625  – 20 August 1672) was a Dutch statesman and mathematician who was a major political figure during the First Stadtholderless Period, when flourishing global trade in a period of rapid European colonial expansion made the Dutch a leading trading and seafaring power in Europe, commonly referred to as the Dutch Golden Age. De Witt was elected Grand Pensionary of Holland, and together with his uncle Cornelis de Graeff, he controlled the Dutch political system from around 1650 until the Rampjaar (Disaster Year) of 1672. This progressive cooperation between the two statesmen, and the consequent support of Amsterdam under the rule of De Graeff, was an important political axis that organized the political system within the republic.

As a leading republican of the Dutch States Party, De Witt opposed the House of Orange-Nassau and the Orangists and preferred a shift of power from the central government to the regenten. However, the Dutch Republic suffered numerous early defeats in the Rampjaar, due to an alliance of England, France, and several German states which planned on invading the Dutch Republic. In the hysteria that followed, he and his brother Cornelis de Witt were blamed and lynched in The Hague, with their corpses at least partially eaten by the rioters. These cannibals were never prosecuted, and some historians claim William of Orange may have incited them.

Johan de Witt was a member of the old Dutch De Witt family. His father was Jacob de Witt, an influential regent from the patrician class in the city of Dordrecht, which in the 17th century was one of the most important cities of the dominating province of Holland. De Witt's mother was Anna van den Corput (1599–1645), niece of Johannes Corputius, an influential Dutch military leader and cartographer. Johan had an older brother, Cornelis de Witt, who had a steep career in the shadow of Johan de Witt. His uncle Andries de Witt held the position of Grand Pensionary of Holland between 1619 and 1621. Through the marriage of one of his other uncles to Margaretha of Nassau, daughter of Anna Johanna of Nassau-Siegen, De Witt was a distant relative of William of Orange-Nassau. Another relationship connected him to the Tromps, Maarten and his son Cornelis Tromp, both admirals of the Netherlands.

Johan and his older brother Cornelis grew up in a privileged environment in terms of education, his father having as good acquaintances important scholars and scientists, such as Isaac Beeckman, Jacob Cats, Gerardus Vossius, and Andreas Colvius. Johan and Cornelis both attended the Latin school in Dordrecht, which imbued them with the values of the Roman Republic. After attending the Latin school in Dordrecht, Johan de Witt studied at the Leiden University, where he excelled at mathematics and law. He received his doctorate from the University of Angers in 1645. He practised law in The Hague as an associate with the firm of Frans van Schooten.

In 1655 Johan de Witt married Wendela Bicker, daughter of Johan Bicker and Agneta de Graeff van Polsbroek, who belonged to the inner circle of the powerful Amsterdam oligarchy. Through his marriage, De Witt became a relative of the ruling patrician families Bicker, De Graeff, Hooft, Witsen, Boelens Loen, and Reynst among others. His brothers-in-law also included Amsterdam patrician Pieter de Graeff (who was also his full cousin), high official Gerard Bicker (I) van Swieten, arms dealer Jacob Trip and banker and financier Jean Deutz, all important and loyal political allies of De Witt. Frans Banninck Cocq (captain of Rembrandt's famous painting The Night Watch) became his uncle-in-law and Joachim Irgens von Westervick (between 1666 and 1675 owner of the private Irgens Estate, which was a huge part of Northern Norway) his cousin-in-law as well. Johan and Wendela had four children, three daughters and one son:

After De Witt's death, Pieter de Graeff, husband of his wife Wendela's younger sister Jacoba Bicker, became the guardian of his children.

In 1650 (the year that stadtholder William II, Prince of Orange died) he was appointed leader of the deputation of Dordrecht to the States of Holland and West Friesland. In December 1650, De Witt became the pensionary of Dordrecht as the successor of Nicolaas Ruys. In 1652, at the age of 27, De Witt was faced with a mob of angry demonstrators of sailors and fishermen in the city of Vlissingen. He held this position until July 1653 and was succeeded by Govert van Slingelandt, a distant relative of his.

In July 1653, the States of Holland elected De Witt Grand Pensionary. In making the appointment, De Witt relied on the express consent of Amsterdam headed by burgomaster and regent Cornelis de Graeff. The States of Holland chose him with the express intercession of his later uncle De Graeff. Since Holland was the Republic's most powerful province, he was effectively the political leader of the United Provinces as a whole – , especially during periods when no stadholder had been elected by the States of most Provinces. The raadpensionaris of Holland was often referred to as the Grand Pensionary by foreigners as he represented the preponderant province in the Union of the Dutch Republic. He led the States of the province by his experience, tenure, familiarity with the issues, and use of the staff at his disposal. He was in no manner equivalent to a modern Prime Minister.