Sir William Ellis (1609–1680) was an English lawyer, judge and politician who sat in the House of Commons at various times between 1640 and 1679, and supported the Parliamentary cause in the English Civil War.
Ellis was second son of Sir Thomas Ellis of Grantham, Lincolnshire. He was educated at Christ's College, Cambridge, graduating B.A. in 1627. Having entered Gray's Inn on 6 November 1627 he was called to the bar on 9 February 1634. He represented Boston, Lincolnshire, in the Short parliament of 1640, and also in the Long parliament.
After Pride's Purge Ellis was readmitted to the House of Commons on 4 June 1649. On 24 May 1654 he was appointed solicitor-general. Shortly afterwards he was elected an ancient of his inn. As solicitor-general he took part in the prosecution of those involved in the Gerard's conspiracy: John Gerard, Peter Vowell, and Summerset Fox on the charge of corresponding with Charles Stuart and conspiring to assassinate the Lord Protector Oliver Cromwell. The trial took place in June 1654. Gerard and Vowell pleaded innocent. They were convicted and sentenced to death. Gerard was beheaded and Vowell was hanged (Fox pleaded guilty and was sentenced to transportation).
The same year Ellis was again returned to Parliament for Boston, and in 1656 for Grantham. He was a member of the committee appointed to frame statutes for Durham College in March 1656. In June 1658 he was engaged in the prosecution of John Hewett and John Mordaunt, charged with levying war against the Protector. Hewett was found guilty and Mordaunt acquitted. He was created a baronet by the Lord Protector Oliver Cromwell on or soon after 31 May 1658. The baronetcy passed into oblivion at the Restoration in May 1660 and unlike some others was not renewed.