小威廉·皮特（英语：William Pitt the Younger，1759年5月28日－1806年1月23日），活跃在18世纪晚期、19世纪早期的英国政治家。1783年，他获任首相，时年24岁，时至今日，仍然是英国历史上最年轻的首相。1801年，他辞去首相一职，但在1804年，他再次出任首相，却在1806年任内去世。担任首相期间，他同时兼任财政大臣。民间为了把他和他的父亲，老威廉·皮特区分开来，通常会在他的名字后面加上“小”（the Younger）。1766年，因父亲获封伯爵，他获得了“The Hon”的前序。

William Pitt the Younger (28 May 1759 – 23 January 1806) was a British politician of the late 18th and early 19th centuries. He became the youngest Prime Minister in 1783 at the age of 24. He left office in 1801, but was Prime Minister again from 1804 until his death in 1806. He was also the Chancellor of the Exchequer throughout his premiership. He is known as "the Younger" to distinguish him from his father, William Pitt the Elder, who had previously served as Prime Minister.