The Irish Stock Exchange has been sold for €137m.
Euronext says it's bought 100% of its shares and voting rights.
The company already runs exchanges in Amsterdam, Brussels, Lisbon, London and Paris.
The sale is subject to regulatory approval - and is expected to be completed early next year.
ISE CEO Deirdre Somers said "this is a landmark day in the 224-year history of ISE and a great day for our customers and our people."
"We believe that Euronext is the perfect partner to enable us to achieve our growth ambitions. These will enable our business to build further on its track record of international achievement and capitalise on new market and product opportunities emerging in Europe,” she added.
Euronext CEO and Chairman of the Managing Board Stéphane Boujnah said Euronext is "delighted to welcome Deirdre Somers and her team".
“The Irish Stock Exchange joining Euronext represents a major milestone in the expansion of Euronext’s federal model since its IPO. ISE brings to Euronext leading global positions in debt, funds and ETF listings markets. As part of Euronext, ISE’s growth initiatives will be reinforced with Euronext’s full support," he said.