Butler will join the Atlantic 10 next season instead of waiting until 2013-14, the school announced Tuesday.
"We are grateful to the Atlantic 10 for welcoming us into the fold a year ahead of schedule," Bulldogs athletic director Barry Collier said in a statement. "Ultimately, it was in our best interest to reach an agreement with the Horizon League for immediate departure."
"The unexpected circumstances that precipitated Butler's request to move immediately were unfortunate," A-10 commissioner Bernadette V. McGlade said in a statement, "but we will welcome Butler into the A-10 as a valuable new member."
According to a source with knowledge of the decision, the move was made in reaction to news that the Horizon League would not allow the Bulldogs to compete for any conference titles or in any conference tournaments in their originally scheduled final season.
Horizon League commissioner John LeCrone said Tuesday that the league doesn't have a bylaw preventing a departing member from competing in conference championships like the Colonial Athletic Association.
Old Dominion, which isn't leaving the CAA for CUSA until the fall of 2013 is appealing to the league to have the bylaw overturned, unlike VCU which bolted for the A-10 a year early.
LeCrone said that the agreement with Butler was amicable and that the two sides agreed not to discuss the particulars of the deal, including the fee Butler will pay to leave early. LeCrone declined to answer whether the league took action about Butler not being able to participate in championships next season.
Butler men's basketball coach Brad Stevens wouldn't comment on the reasons for leaving early but did say that he is "excited about moving into a great new league."
Meanwhile, LeCrone said the Horizon League will play 16 basketball games next season with nine teams instead of 18 with 10. LeCrone said the league will have a true-round robin schedule and be fine next season. He said the league will look at expansion but made not overtures as to what direction the Horizon League would move in the near future.
The change in plans now means both the A-10 and Butler must alter their men's basketball schedules.
Stevens said the Bulldogs now need to schedule three more games for next season. Butler had a series with Xavier that will have to be replaced since they now are league partners.
McGlade said the league will figure out a one-year, 16-team schedule soon. The league meetings wrapped up last week without an official decision on a 15-team schedule so the league still has time to sort out the scheduling.
Butler will join VCU, also coming in in 2012-13, giving the A-10 16 teams for the 2012-13 season. The conference will go down to 14 when Temple (Big East) and Charlotte (Conference USA) leave the following year.
Butler initially announced its move from the Horizon League to the A-10 in early May.