The 79-year-old Selig has repeatedly said since 2003 that his retirement was imminent, but Thursday marked the first time he issued a formal statement.
He says he will announce a transition plan shortly that will include a reorganization of central baseball management.
Selig bought the Seattle Pilots in bankruptcy court in 1970 and moved the team to Milwaukee. He was part of the group that forced Fay Vincent's resignation.
Selig took over as acting commissioner on Sept. 9, 1992, in his role as chairman of the executive council. He repeatedly said he would not take the job full time but was formally elected commissioner July 9, 1998. He agreed to new contracts in 2001, 2004, 2008 and 2012.
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