Marquess of Ailsa, of the Isle of Ailsa in the County of Ayr, is a title in the Peerage of the United Kingdom. It was created on 10 September 1831 for Archibald Kennedy, 12th Earl of Cassillis. The title Earl of Cassillis (pronounced "Cassels") had been created in 1509 for the 3rd Lord Kennedy. This title had been created in the Peerage of Scotland in 1457. The 1st Marquess had been created Baron Ailsa in the Peerage of the United Kingdom on 12 November 1806.
The name of the title was taken from the Island of Ailsa Craig in the Firth of Clyde. The island is still owned by the family but it is currently up for sale for £1,500,000.
James Kennedy, Archbishop of St Andrews, was the younger brother of the first Lord Kennedy.
The Marquess of Ailsa is the hereditary Clan Chief of Clan Kennedy.
The family seat was Cassillis House, Maybole, Ayrshire, sold in 2009.
Lords Kennedy (1457) 
Earls of Cassilis (1509) 
- David Kennedy, 1st Earl of Cassilis (d. 1513)
- Gilbert Kennedy, 2nd Earl of Cassilis (d. 1527)
- Gilbert Kennedy, 3rd Earl of Cassilis (1515–1558)
- Gilbert Kennedy, 4th Earl of Cassilis (c. 1541–1576)
- John Kennedy, 5th Earl of Cassilis (c. 1573–1615)
- John Kennedy, 6th Earl of Cassilis (d. 1668)
- James Kennedy, Lord Kennedy (1642–between 1642 and 1643)
- John Kennedy, 7th Earl of Cassilis (d. 1701)
- John Kennedy, Lord Kennedy (c. 1672–1700)
- John Kennedy, 8th Earl of Cassilis (1700–1759)
- Thomas Kennedy, 9th Earl of Cassilis (d. 1775)
- David Kennedy, 10th Earl of Cassilis (d. 1792)
- Archibald Kennedy, 11th Earl of Cassilis (d. 1794)
- Archibald Kennedy, 12th Earl of Cassilis (1770–1846) (created Marquess of Ailsa in 1831)
Marquesses of Ailsa (1831) 
The heir presumptive is the present holder's younger brother Lord David Thomas Kennedy (b. 1958) married with a son.