M&S has bought 50% of Marks & Spencer Marinopoulos BV, which operates 38 M&S stores in Greece, Romania, Bulgaria and Switzerland, for €50 million (£38m).
The joint venture plans to open up to 50 new stores in these markets over the next few years.
The deal is part of the UK retailer's plans to invest in existing franchise partners to speed up international expansion, and increase operating efficiency.
M&S plans to grow its international business to between 15% and 20% of group revenue in the next five years.
M&S director of international business Carl Leaver said: "We have an exciting opportunity to open many more Marks & Spencer stores in Greece and the Balkans and our joint venture with Marinopoulos will mean we can really put M&S on the map in this part of the world.
"In Greece, we already have a solid platform to build on an there is a growing demand for great quality, good value M&S product in markets like Romania and Bulgaria. We have worked with Marinopoulos for over 30 years and their retailer expertise and local knowledge make them the ideal partner for us."