Louisa Petty, Marchioness of Lansdowne (; later Petty Fitzmaurice; 1755 – 7 August 1789), known as the Countess of Shelburne from 1779âÂÂ84, was an Anglo-Irish aristocrat from the Mac Giolla Phádraig dynasty. She was the wife of Prime Minister William Petty, 2nd Earl of Shelburne.
The Marchioness was the youngest daughter of John FitzPatrick, 1st Earl of Upper Ossory, and his wife, Lady Evelyn (née Leveson-Gower; daughter of John Leveson-Gower, 1st Earl Gower).
Her eldest brother was John FitzPatrick, 2nd Earl of Upper Ossory. She had a younger brother Richard, who also became a noted statesman and soldier, and an elder sister, Lady Mary Fox (Baroness Holland), the subject of paintings by Thomas Gainsborough, Sir Joshua Reynolds, and Pompeo Batoni.
She married, as his second wife, William Petty, 2nd Earl of Shelburne, who served as Prime Minister for 10 months in 1782–83, and was afterwards created Marquess of Lansdowne. The couple were married on 8 July 1779
She died on 7 August 1789.