William Pitt (28 May 1759 – 23 January 1806) was a British statesman who served as the last prime minister of Great Britain from 1783 until the Acts of Union 1800, and the first official prime minister of the United Kingdom from January 1801. He left office in March 1801, but served as prime minister again from 1804 until his death. He was also Chancellor of the Exchequer for all his time as prime minister. He is known as "Pitt the Younger" to distinguish him from his father, William Pitt the Elder, who had also previously served as prime minister.

Pitt's premierships, which came during the reign of King George III, were dominated by major political events in Europe, including the French Revolution and the Napoleonic Wars. Pitt, although often referred to by historians as a Tory, or "new Tory", called himself an "independent Whig" and was generally opposed to the development of a strict partisan political system.

Pitt was regarded as an outstanding administrator who worked for efficiency and reform, bringing in a new generation of competent administrators. He increased taxes to pay for the great war against France and cracked down on radicalism. To counter the threat of Irish support for France, he engineered the Acts of Union 1800 and tried (but failed) to secure Catholic emancipation as part of the Union. He created the "new Toryism", which revived the Tory Party and enabled it to stay in power for the next quarter of a century.

The historian Asa Briggs argues that his personality did not endear itself to the British mind, for Pitt was too solitary and too colourless, and too often exuded an attitude of superiority. His greatness came in the war with France. Pitt reacted to become what Lord Minto called "the Atlas of our reeling globe". William Wilberforce said, "For personal purity, disinterestedness and love of this country, I have never known his equal." The historian Charles Petrie concludes that he was one of the greatest prime ministers "if on no other ground than that he enabled the country to pass from the old order to the new without any violent upheaval. ... He understood the new Britain." For this he is ranked highly amongst all British prime ministers in multiple surveys.

Pitt served as prime minister for a total of eighteen years, 343 days, making him the second-longest-serving British prime minister after Robert Walpole. Having entered office at the age of 24, Pitt is the youngest prime minister in both British and world history.

William Pitt, the second son of William Pitt, 1st Earl of Chatham, was born on 28 May 1759 at Hayes Place in the village of Hayes in Kent. He was from a political family on both sides, as his mother, Hester Grenville, was the sister of the former prime minister George Grenville. According to the biographer John Ehrman, Pitt exhibited the brilliance and dynamism of his father's line, and the determined, methodical nature of the Grenvilles.

Suffering from occasional poor health as a boy, he was educated at home by the Reverend Edward Wilson. An intelligent child, Pitt quickly became proficient in Latin and Greek. He was admitted to Pembroke College, Cambridge, on 26 April 1773, a month before turning fourteen, going up to Cambridge in October 1773. He studied political philosophy, classics, mathematics, trigonometry, chemistry and history. At Cambridge, Pitt was tutored by George Pretyman Tomline, also at Pembroke College, who became a close personal friend and looked after Pitt whilst at University. Pitt later appointed Pretyman Bishop of Lincoln, then Winchester, and drew upon his advice throughout his political career. While at Cambridge, he befriended the young William Wilberforce, who became a lifelong friend and political ally in Parliament. Pitt tended to socialise only with fellow students and others already known to him, rarely venturing outside the university grounds. Yet he was described as charming and friendly. According to Wilberforce, Pitt had an exceptional wit along with an endearingly gentle sense of humour: "no man ... ever indulged more freely or happily in that playful facetiousness which gratifies all without wounding any." An example of Pitt's innocent wit was recorded by Sir John Sinclair. In the early years of Pitt's ministry there was great interest in the new young First minister. Sinclair was required to write an account of Pitt to satisfy foreign curiosity when he was abroad. At the end of a long description of Britain's eminent leader he added: ‘Of all the places where you have been, where did you fare best?’ My answer was, ‘In Poland; for the nobility live there with uncommon taste and splendour; their cooks are French,- their confectioners Italian, - and their wine Tokey.’ He immediately observed, ‘I have heard before of The Polish diet.’

In 1776 Pitt, plagued by poor health, took advantage of a little-used privilege available only to the sons of noblemen, and chose to graduate without having to pass examinations. Pitt's father was said to have insisted that his son spontaneously translate passages of classical literature orally into English, and declaim impromptu upon unfamiliar topics in an effort to develop his oratorical skills. Pitt's father, who had by then been raised to the peerage as Earl of Chatham, died in 1778. As the younger son, Pitt received only a small inheritance. In the months following the death of the Earl of Chatham, Pitt was forced to defend his father's reputation. This came about when the Bute family made claims that the late Lord had sought out the Earl of Bute with the desire to form a political coalition. Pitt although just over nineteen years of age publicly argued that this was not the case. Faced with Pitt's arguments the Bute family backed off and ceased making their claims. He acquired his legal education at Lincoln's Inn and was called to the bar in the summer of 1780.