MILAN, July 1 (Reuters) - Italy's Agnelli family, whose investments include carmaker Stellantis and soccer club Juventus, is set to buy a slice of French healthcare group Institut Merieux for 833 million euros ($867 million) through its Exor holding company.

The deal, announced on Friday by the two companies, will see privately-held Institut Merieux selling a 10% stake to Exor through a reserved capital increase.

It marks the first major international move into the healthcare business by Exor, which will soon have 9 billion euros to invest after it completes the sale of Bermuda-based reinsurer Partner Re.

CEO John Elkann, the scion of the Agnelli family, has said Exor's new investments would focus on the healthcare, technology and luxury sectors. In luxury, Exor already owns 24% in high-end shoemaker Louboutin and is the largest shareholder in Chinese group Shang Xia.

"For us, this is a unique opportunity to enter the healthcare sector with a strong partner with whom we share the same values and long-term horizon," Elkann said in the statement.

Lazard advised Lyon-based Institut Merieux on the deal alongside French lawyers Juris Vendome and Gide Loyrette Nouel, while Exor was represented by Paris-based law firm De Pardieu Brocas Maffei.

Founded 125 years ago, Institut Merieux employs over 21,000 people globally, with a turnover of 4 billion euros. It controls five companies in different healthcare segments, including Paris-listed bioMerieux, whose market capitalisation tops 11 billion euros.

The deal, which values Institut Merieux at over 8 billion euros, is expected to be completed during the summer. Exor will pay one third of the agreed amount at the closing of the deal and the remainder within the following 12 months.

Elkann and Exor partner Benoit Ribadeau Dumas will join Institut Merieux's board.

Exor - which is the single largest shareholder in Stellantis and has controlling stakes in companies including sports car maker Ferrari and Serie A soccer team Juventus - earlier this year bought a 45% stake in Italy's Lifenet Healthcare for 67 million euros.

($1 = 0.9611 euros) (Reporting by Giulio Piovaccari Additional reporting by Pamela Barbaglia Editing by Giulia Segreti and Mark Potter)