Selig, 79, took over as interim commissioner in 1992, when Fay Vincent was voted out by the owners, and became the full-time commissioner in 1998. He was ninth commissioner of Major League Baseball. Only the first commissioner -- Kenesaw Mountain Landis who served from 1920-44 -- held the position longer.
"It remains my great privilege to serve the game I have loved throughout my life," Selig said Thursday. "Baseball is the greatest game ever invented and I look forward to continuing its extraordinary growth and addressing several significant issues during the remainder of my term."
Selig also thanked the owners for "unwavering support" and players "who give me unlimited enthusiasm about the future of our game."
"Together we have taken this sport to new heights and have positioned our national pastime to thrive for generations to come," he said. "Most of all, I would like to thank our fans, who are the heart and soul of our game."
Among changes Selig is credited with is the split of the American and National leagues into three divisions, the institution of wild-card playoff games and interleague play.
He also ordered that the outcome of the All-Star Game determine the home-field advantage for the World Series, the use of limited instant replay and expanded programs related to ending performance-enhancing drug use.
But Selig also was in office when labor issues hit baseball, leading him to cancel the World Series in 1994, although that did bring the owners and players into a strong labor negotiation relationship.