Among the features of the deal -- which was successfully negotiated Nov. 22 -- will be a move by the Houston Astros from the National League Central to the American League West.
That will put five teams in each of baseball's six divisions and will create games between the two leagues throughout the season. Houston's move will take place in 2013.
"I am very pleased that the clubs have given their unanimous support of the new agreement," Selig said. "Our sport has taken many significant strides over the past decade and this agreement will allow us to continue to grow the game."
A second wild-card team from each league will be added to the playoffs, possibly as soon as 2012.
The use of instant replay will be expanded under the contract, to include determining whether a ball is fair or foul or whether a ball is caught cleanly by a fielder or has been trapped.