William Lowther, 1st Earl of Lonsdale KG (29 December 1757 – 19 March 1844) was a British Tory politician and nobleman.
Lowther was the son of Sir William Lowther, 1st Baronet, of Little Preston and Swillington, and his wife Anne Zouch. He was educated at Westminster and Trinity College, Cambridge.
Lowther was briefly Member of Parliament for Appleby in 1780, for Carlisle from 1780 to 1784 and for Cumberland from 1784 to 1790. In 1796, he was returned as Member of Parliament for Rutland, holding the seat until 1802. In that year, he inherited by special remainder the titles of Viscount Lowther and Baron Lowther from his third cousin once removed, the Earl of Lonsdale of the first creation, as well as his immense estates. He was also appointed to the northern Lord Lieutenancies of Cumberland and Westmorland. In 1807, Lowther was himself created Earl of Lonsdale and appointed a Knight of the Garter.
A coal magnate, he spent £200,000 on the Lowther estate and built a new Lowther Castle. A Tory in politics, he seems to have been tolerant and well-liked, disdaining sabbatarianism and serving as patron for a number of painters and authors, including William Wordsworth. Lonsdale died at York House, Twickenham in 1844. Lowther also enjoyed fox hunting, serving as Master of the Cottesmore Hunt from 1788 to 1802 and 1806 to 1842.