William Lowther, 1st Earl of Lonsdale, KG, 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.

Lowther Castle is a crenellated country house in the historic county of Westmorland, which now under the current unitary authority of Westmorland and Furness, in the ceremonial county of Cumbria, England. The estate has belonged to the Lowther family, latterly the Earls of Lonsdale, since the Middle Ages. It is a fully managed ruin, open to visits by the public to the shell of the castle and some of the gardens since 2011. Additional work was completed since that time, most recently on the extensive gardens.