The swap creates an estimated $10-plus million in long-term savings for the Nets as they continue to reshape their team after trading another mainstay -- point guard Jason Kidd -- to Dallas in February.
Milwaukee has been looking to rid itself of several bloated contracts and decided it was worth trading the rookie that former general manager Larry Harris selected last year along with the rest of Simmons' contract. Simmons has two years and a little more than $20 million left on his deal.
Harris was let go in March, and new general manager John Hammond is looking to fill several spots after the team tumbled to a 26-56 finish. Jefferson provides veteran leadership to a team desperately seeking winners in the locker room.
"Richard Jefferson is a hardworking, proven, dynamic performer at the forward position," Hammond said. "He's an aggressive player with terrific credentials over his career. He's been a winner, having gone to the playoffs in six of his seven seasons, and we're excited to bring him to Milwaukee."
The trade also enables Yi to make a move to the larger media market he was hoping for this time last year, when he was taken with the sixth overall pick. Kiki Vandeweghe, New Jersey's new general manager alongside team president Rod Thorn, is a longstanding Yi fan who has frequently likened the 7-footer's perimeter skills and potential to Dallas Mavericks forward Dirk Nowitzki.
"We certainly gave up a good player, but to get a good player you have to give up a good player. With Yi, we feel his upside is very up," Thorn said. "We feel strongly he's going to be a real good player for us.
"We feel that Yi can be a very special player. He is a 20-year-old 7-footer who shoots the ball extremely well, and he is an excellent addition to our frontcourt."
Yi took his time signing with Milwaukee after being selected in last year's draft and wasn't swayed to join Milwaukee until a contingent that included owner and U.S. Sen. Herb Kohl traveled to China to complete the deal.
Yi made 49 starts and played in 66 games, averaging 8.6 points and 5.2 rebounds. He vied with Charlie Villanueva for playing time.
Simmons never lived up to a five-year, $47 million contract he signed in the offseason after being honored as the NBA's Most Improved Player in 2005. After a tough first year adjusting, he missed the entire 2006-07 season with ankle and foot injuries and averaged just 7.6 points in 70 games last season.
"He's greatly appreciative of everything the Bucks have done for him. They gave him a terrific contract, a great opportunity. He's had some injuries, which have been really unfortunate," Simmons' agent, Mark Bartelstein, said. "In tough times, you need a fresh start to kind of change your luck."
The Bucks are also supplying new coach Scott Skiles with a veteran player coming off his best season to fill a problem position. Jefferson averaged 22.7 points per game last season and played all 82 games after an injury-plagued 2006-07 season.
The Nets get cap room and rid themselves a large contract, too. Jefferson, the second-leading scorer in Nets history, averaged 22.6 points last season and has three years and more than $42 million left on his contract.
"I want to thank Richard for his contributions to the Nets over the past seven years," Thorn said. "He always conducted himself in a professional manner, and we wish him only the best for the remainder of his career."
The move continues the rebuilding of the franchise in the post-Kidd era and puts New Jersey in position to be a major player in the 2010 offseason, which is expected to include a bumper crop of free agents such as LeBron James.
Milwaukee's starting lineup also looks much clearer after the trade. Mo Williams will start at the point, Michael Redd at shooting guard, Jefferson and Villanueva at forward and Andrew Bogut, the No. 1 pick in 2005, at center.
ESPN.com senior NBA writer Marc Stein contributed to this report. Information from The Associated Press was used in this report.