Texas has confirmed Papapetrou's departure, which was reported earlier by Eurohoops.net.
Olympiacos also posted a story on its website that Papapetrou, a native of Athens, has inked a five-year deal. A source told ESPN.com the deal was worth approximately $2 million.
The 6-foot-8, 225-pound Papapetrou's departure is the latest hit for coach Rick Barnes and the Longhorns' program, which has lost Avery Bradley, Jordan Hamilton, Tristan Thompson and Cory Joseph early to the NBA and also watched as Sheldon McClellan, Julien Lewis and Jaylen Bond all transferred out of the program following last season.
Papapetrou had spurned an earlier offer to play professionally in Greece. But according to a source close to the situation, Olympiacos increased the offer in the past few days as Papapetrou was preparing to return to Austin.
Papapetrou averaged 8.3 points and 4.4 rebounds last season, but was set to be the key piece for a young Texas team this season. He was a mismatch at both forward spots, and showed NBA potential recently while playing for the Greek national team at the U-20 European Championships.
Texas finished 16-18 overall last season and seventh in the Big 12 with a 7-11 mark. It was the first time in Barnes' 15 seasons in Austin that the Longhorns failed to reach the NCAA tournament.