James Grenville, 1st Baron Glastonbury

James Grenville, 1st Baron Glastonbury, PC (6 July 1742 – 26 April 1825) of Butleigh Court, Somerset was a United Kingdom politician, who was a member of both houses of Parliament during his career.

The Grenvilles were the most prominent aristocratic family in the south-eastern English county of Buckinghamshire in the 18th and early 19th centuries.

He sat for his family borough of Buckingham 1770–1790, from 1774 to 1780 in partnership with his twin brother, Richard.

As a politician he generally followed his family connection up to 1801 and after 1806, but between those years he continued to support William Pitt the Younger instead of becoming closer to Charles James Fox as most of his politically active Grenville relatives did.

William Petty, 2nd Earl of Shelburne offered to make Grenville Chancellor of the Exchequer or Secretary at War, but he declined these appointments.