Marquess of Bath is a title in the Peerage of Great Britain. It was created in 1789 for Thomas Thynne, 3rd Viscount Weymouth. The Marquess holds the subsidiary titles Baron Thynne, of Warminster in the County of Wiltshire, and Viscount Weymouth, both created in 1682 in the Peerage of England. He is also a baronet in the Baronetage of England.

Thomas Thynne, 1st Marquess of Bath, KG, PC, of Longleat in Wiltshire, was a British politician who held office under King George III. He served as Southern Secretary, Northern Secretary and Lord Lieutenant of Ireland. Between 1751 and 1789, he was known as the 3rd Viscount Weymouth. He is possibly best known for his role in the Falklands Crisis of 1770.