MLB announced the move Friday, adding two clubs to the postseason format.
A second wild-card team now will be included in each league. There then will be a one-game playoff to decide which team advances.
Commissioner Bud Selig says the change "increases the rewards of a division championship and allows two additional markets to experience playoff baseball each year."
This is the first change in baseball's playoff structure since the 1995 season, when wild-card teams were first added.