Lewis said in a statement that he took an over-the-counter supplement late last season that included a substance he did not realize was banned by the NBA.
"First and foremost I take full responsibility for the situation and accept the corresponding penalty," Lewis said. "I apologize to Magic fans, my teammates and this organization for not doing the research that should come with good judgment."
He will be suspended without pay for the first 10 regular-season games for which he is eligible and physically able to play. He will be docked about $1.6 million of his $18 million salary for the 2009-10 season.
The Orlando Sentinel first reported the positive test Thursday.
Magic coach Stan Van Gundy reached out to Lewis when he learned of the suspension, according to the Sentinel. "He's one of the best people in this league and has been a model citizen and a great team guy and simply made a very innocent mistake," Van Gundy told the newspaper.
"I just wanted to make sure that he knew that he has the full support of the coaching staff and everybody in the organization, including, I'm sure, his teammates," Van Gundy said, according to the Sentinel.
ESPN.com's Jackie MacMullan reported that according to league sources, Lewis tested positive for steroids precursor DHEA and asked for a retest, as is his right under the collective bargaining agreement.
Lewis did not believe he was taking anything illegal, so he sat down with the Magic staff and they isolated an over-the-counter supplement he was taking that contained DHEA, banned by The National Basketball Players Association. It is frequently used to combat fatigue. Lewis had not told Magic officials he was taking it. He was randomly tested again during the Finals, but the DHEA was still in his system, MacMullan reported.
Lewis is the sixth player to test positive for performance-enhancing drugs since the league began testing in 1999.
"I hope every athlete can learn from my mistake that supplements, no matter how innocent they seem, should only be taken after consulting an expert in the field," Lewis said in his statement.
Lewis' agent, Tony Dutt, did not immediately respond to a request for comment.
The 29-year-old Lewis was an All-Star last season and is one of the NBA's best 3-point shooters at 6-foot-10, allowing him to play on the perimeter and the Magic to create the matchup problems that helped them beat Boston and Cleveland in the postseason.
He was hurt late in the regular season, missing the final three games with right knee tendinitis, then returned to average 19.0 points in the playoffs to help lead the Magic to the NBA Finals.
Information from The Associated Press was used in this report.