William Lenthall (1591 – 3 September 1662) was an English politician of the Civil War period. He served as Speaker of the House of Commons for a period of almost twenty years, both before and after the execution of King Charles I.

He is best remembered for his defiance of the king on 4 January 1642 when Charles entered the chamber of the House of Commons, supported by 400 armed men, in an attempt to seize five members whom he accused of treason. When Charles asked Lenthall where the five were, Lenthall famously replied "I have neither eyes to see nor tongue to speak in this place but as this House is pleased to direct me". It was the first time in English history that a speaker of the House of Commons had declared his allegiance to the liberty of parliament rather than the will of the monarch.

Lenthall was born in Henley-on-Thames, Oxfordshire, the second son of William Lenthall (died 1596) and Frances Southwell. His ancestors had migrated from Herefordshire to Oxfordshire in the 15th century. The family was recusant under Queen Elizabeth I, but this branch became Protestant after the early death of Lenthall's father in 1596. Lenthall's mother, Frances (sister of the Jesuit priest and poet Robert Southwell), conformed to the established Church.

Lenthall was educated at Lord Williams's School in Thame. He matriculated at St Alban Hall, Oxford in 1607 but left in 1609 without taking a degree. He moved to Lincoln's Inn and was called to the bar in 1616, becoming a bencher of the inn in 1633. He built up a successful legal practice, becoming recorder of Woodstock in 1621, an Oxfordshire magistrate in 1631, and recorder of Gloucester in 1638.

Lenthall's parliamentary career began in 1624 when he sat as member for New Woodstock in Oxfordshire. He failed to be re-elected in 1625, but again represented the constituency during the 1640 Short Parliament, on several occasions being called upon to chair grand committees of the House on important subjects, including ship money and parliamentary grievances. The Short Parliament was dissolved on 5 May 1640 after only three weeks.

When Charles I recalled parliament once more on 4 November 1640, at the start of what became known as the Long Parliament, Lenthall again attended on behalf of New Woodstock. Discovering that his preferred candidate for speaker, Sir Thomas Gardiner, had failed to be returned, the king reviewed the list of available lawyers and approved Lenthall as the new speaker, a position that Lenthall was to hold for most of the next twenty years.

From the start, Lenthall had his critics. Sir Henry Mildmay criticised him for letting too many speak during a debate, he was accused of partiality and procedural errors, and at one point was made to look foolish over a point of precedence. However, the journal of Sir Simonds d'Ewes (who was not generally supportive) suggests that in the opening months of the Long Parliament Lenthall was very much in control of proceedings.

During 1640 and 1641 Lenthall proved himself a competent speaker. He introduced or codified a variety of procedural rules including the establishment of the duration of parliamentary privilege before and after sittings, the imposition of a penalty for speaking when another member had the floor, and the rule that while one piece of business was before the House a motion on another could not be made.