The positive test violates the terms of the NBA/NBPA Anti-Drug Program.
"During my injury rehab process, before I signed with the Nets, I was prescribed a treatment that included small doses of a substance recently added to the NBA's prohibited substance list," Chandler said in a statement to ESPN. "I did not realize this substance was banned, and neither did the doctor.
"I accept responsibility and apologize to my Nets teammates, coaches, front office and fans for this mistake. I will continue to work hard to prepare for the upcoming season."
Chandler, 32, averaged 6.0 points and 4.2 rebounds while playing in 51 games for the Philadelphia 76ers and LA Clippers last season. Despite the suspension, he will be allowed to participate during training camp and play in preseason games for the Nets.
The Nets signed Chandler as a free agent in July, with an expectation that he'd play a significant role in the team's frontcourt rotation.
The 25-game suspension will result in a loss of $582,898 -- $23,316 per game -- for Chandler, according to ESPN's Bobby Marks.
Chandler will start the regular season on the active list and move to the suspension list after the fifth game (Nov. 1), which would allow the Nets, who have 15 guaranteed contracts, to add a 16th player after that game.
Brooklyn plays the following night, Nov. 2, at Detroit.
Chandler will be eligible to be activated from the suspension list for the Dec. 15 game vs. Philadelphia.
In 11 seasons, Chandler, a first-round pick of the New York Knicks in 2007, has averaged 12.9 points and 5.3 rebounds.