William Lowther, 1st Earl of Lonsdale, KG (29 December 1757 – 19 March 1844), also known as Sir William Lowther, 2nd Baronet, of Little Preston, from 1788 to 1802, and William Lowther, 2nd Viscount Lowther, from 1802 to 1807, was a British Tory politician and nobleman known for building Lowther Castle.