| Honorific Prefix: | The Right Honourable |
| The Countess of Bute | |
| Birth Date: | 19 January 1718 |
| Birth Place: | Constantinople, Ottoman Empire |
| Birth Name: | Mary Wortley Montagu |
| Death Place: | Isleworth, England |
| Resting Place: | St Leonard's Churchyard, Wortley, South Yorkshire, England |
| Nationality: | British |
| Known For: | Spouse of the prime minister of Great Britain (1762–1763) |
| Parents: | Sir Edward Wortley Montagu Lady Mary Pierrepont |
| Children: | 11, including: John Stuart, 1st Marquess of Bute Hon. James Stuart Hon Frederick Stuart Hon. Charles Stuart Hon. William Stuart Lady Louisa Stuart |
Mary Stuart, Countess of Bute, 1st Baroness Mount Stuart (; 19 January 1718 – 6 November 1794) was the wife of British nobleman John Stuart, 3rd Earl of Bute, who served as Prime Minister from 1762 to 1763.
Lady Bute was born in 1718, the only daughter of Sir Edward Wortley Montagu and Lady Mary Pierrepont, the daughter of Evelyn Pierrepont, 1st Duke of Kingston-upon-Hull. She was born during her father's tenure as ambassador to the Ottoman Empire, which her mother wrote about in her Letters from Turkey.
On 24 August 1736, she married John Stuart, 3rd Earl of Bute, who became the prime minister of Great Britain in 1762.[1] The couple had five sons and six daughters, including:
In 1761, she was created Baroness Mount Stuart, of Wortley in the county of York, with a remainder to her male heirs by her husband.
Lady Bute died on 6 November 1794 in Isleworth, Middlesex.[3] Her eldest son, John, succeeded to her title.
In 1774, Mary Delany wrote to her brother Bernard Granville, Jacobite Duke of Albemarle, saying: "You know so much of Lady Bute that I need say nothing of her agreeableness, her good sense, and good principles, which with great civility must be always pleasing."[4]
Writing for the Oxford Dictionary of National Biography, Karl Wolfgang Schweizer said that: "Lady Bute seems to have been a woman of prudence, loyalty, and tact, greatly devoted to her husband and family."