Charles Louis de Secondat, baron de La Brède et de Montesquieu (18 January 1689 – 10 February 1755), generally referred to as simply Montesquieu, was a French judge, intellectual, historian, and political philosopher.

He is the principal source of the theory of separation of powers, which is implemented in many constitutions throughout the world. He is also known for doing more than any other author to secure the place of the word despotism in the political lexicon. His anonymously published The Spirit of Law (De l'esprit des lois, 1748) first translated into English (Nugent) in a 1750 edition was received well in both Great Britain and the American colonies, and influenced the Founding Fathers of the United States in drafting the U.S. Constitution.

Montesquieu was born at the Château de la Brède in southwest France, 25 kilometres (16 mi) south of Bordeaux. His father, Jacques de Secondat (1654–1713), was a soldier with a long noble ancestry, including descent from Richard de la Pole, Yorkist claimant to the English crown. His mother, Marie Françoise de Pesnel (1665–1696), who died when Charles was seven, was an heiress who brought the title of Barony of La Brède to the Secondat family.

His family was of Huguenot origin. After the death of his mother he was sent to the Catholic College of Juilly, a prominent school for the children of French nobility, where he remained from 1700 to 1711. His father died in 1713, and he became a ward of his uncle, the Baron de Montesquieu. In 1714, he became a counselor of the Bordeaux Parlement. He showed a preference for Protestantism.

In 1715 he married the Protestant Jeanne de Lartigue, with whom he eventually had three children. The Baron died in 1716, leaving him his fortune as well as his title, and the office of président à mortier in the Bordeaux Parlement, a post that he held for twelve years.

Montesquieu's early life was a time of significant governmental change. England had declared itself a constitutional monarchy in the wake of its Glorious Revolution (1688–1689), and joined with Scotland in the Union of 1707 to form the Kingdom of Great Britain. In France, the long-reigning Louis XIV died in 1715, and was succeeded by the five-year-old Louis XV. These national transformations had a great impact on Montesquieu, and he referred to them repeatedly in his work.

Montesquieu eventually withdrew from the practice of law to devote himself to study and writing. He achieved literary success with the publication of his 1721 Persian Letters (French: Lettres persanes), a satire representing society as seen through the eyes of two Persian visitors to Paris, cleverly criticizing absurdities of contemporary French society. The work was an instant classic and accordingly was immediately pirated.

In 1722, he went to Paris and entered social circles with the help of friends including the Duke of Berwick whom he had known when Berwick was military governor at Bordeaux. He also acquainted himself with the English politician Viscount Bolingbroke, some of whose political views were later reflected in Montesquieu's analysis of the English constitution. In 1726 he sold his office, bored with the parlement and turning more toward Paris. In time, despite some impediments he was elected to the Académie Française in January 1728.