The Brooklyn Nets superstar holds the option to buy an additional 5% "in the near future."
"I'm excited to partner with the Philadelphia Union for years to come. My team and I connected instantly with the Union coaching staff and leadership, as well as the team's story," Durant said.
A two-time NBA champion and 10-time All-Star, Durant joins the likes of James Harden and Russell Wilson with his investment in MLS. The Houston Rockets guard purchased a minority stake in the Dynamo last year, while the Seattle Seahawks quarterback did the same with the Sounders.
The deal also includes an expanded partnership with Thirty Five Ventures, the company Durant and sports business executive Rich Kleiman run together. Along with the Kevin Durant Charity Foundation (KDCF), the club will aim to "grow the Union's footprint" worldwide through that part of the deal.
"I've been a fan of the sport, and then seeing how fast the popularity of the league was growing, seeing more fans pop up in different cities around the country, and then seeing how these franchises impact the city's businesses and people individually was very intriguing," Durant told Jeff Carlisle of ESPN.
Jay Sugarman, the Union's majority owner, says the team is worth over $325 million, according to Carlisle. That valuation means Durant's stake comes in at a minimum of $16.25 million.