Marquess of Sligo is a title in the Peerage of Ireland created in 1800 for the Earl of Altamont.

The Marquess holds the subsidiary titles of Earl of Altamont (1771), Earl of Clanricarde (1800), Viscount Westport (1768), Baron Mount Eagle of Westport (1760), and Baron Monteagle (1806), all but the last in the Peerage of Ireland.

Prior to the passage of the House of Lords Act 1999, the Marquess sat in the House of Lords as Baron Monteagle in the Peerage of the United Kingdom.

Earls of Altamont (1771)

• John Browne, 1st Earl of Altamont (d. 1776)
• Peter Browne, 2nd Earl of Alamont (d. 1780)
• John Denis Browne, 3rd Earl of Altamont (1756-1809), elected a Representative Peer in 1800, created Marquess of Sligo later that year

Marquesses of Sligo (1800)

• John Denis Browne, 1st Marquess of Sligo (1756-1809)
• Howe Peter Browne, 2nd Marquess of Sligo (1788-1845)
• George John Browne, 3rd Marquess of Sligo (1820-1896)
• John Thomas Browne, 4th Marquess of Sligo (1824-1903)
• Henry Ulick Browne, 5th Marquess of Sligo (1831-1913)
• George Ulick Browne, 6th Marquess of Sligo (1856-1935)
• Ulick John Browne, 7th Marquess of Sligo (1898-1941)
• Arthur Howe Browne, 8th Marquess of Sligo (1867-1951)
• Terence Morris Browne, 9th Marquess of Sligo (1873-1952)
• Denis Edward Browne, 10th Marquess of Sligo (1908-1991)
• Jeremy Ulick Browne, 11th Marquess of Sligo (b. 1939)