Butler will officially join the Atlantic 10 for the 2013-14 season on Wednesday, a source with direct knowledge of the negotiation process told ESPN.com on Tuesday night.
The Bulldogs will leave the Horizon League in all sports and join the A-10 after it formally seeks admission to the league. The A-10 will then accept the Bulldogs' application to make it official.
Without saying where Butler is headed, the Horizon League issued a statement Wednesday acknowledging that Butler is leaving the conference.
"We are proud of the role the Horizon League played in providing a platform for Butler to significantly improve its athletics programs and achieve the highest level of national competitiveness in men's basketball," Horizon League commissioner Jon LeCrone said in the statement.
"We are committed to sustaining those same opportunities for all League members. Under the leadership of our board of directors, we have an aggressive growth agenda which has been in the planning stages for some time."
Butler will replace Temple and become the A-10's 14th member for the 2013-14 season. Temple is leaving the A-10 in 2013-14 for the Big East in all sports. Temple's football program is joining the Big East in fall 2012.
The move ends months of speculation that the Bulldogs would join the league, a coup for A-10 commissioner Bernadette McGlade in her first major decision since becoming head of the conference.
Butler is a two-time national runner-up under coach Brad Stevens in 2010 and '11. The Bulldogs, who missed the NCAA tournament this past season, have been the most dominant program in the Horizon League recently.
Butler's Indianapolis home helps the A-10 continue its migration to the Midwest, linking the Bulldogs with league members Xavier, Dayton, Duquesne and Saint Louis.
The rest of the league includes strong members UMass, Richmond, Saint Joseph's and Saint Bonaventure as well as Rhode Island, George Washington, La Salle and Fordham.
The one question mark is Charlotte. The 49ers were offered a spot in the Sun Belt, but rejected it, according to a source. The same source said Charlotte is expected to join Conference USA for all sports since it started a football program. A source with knowledge of the A-10's plans said the league expects Charlotte to leave, possibly as early as 2013 but it could be 2014.
The source said the A-10 has had talks with Colonial Athletic Association members George Mason and VCU, but the latter is on hold with the departure of athletic director Norwood Teague to the same position to Minnesota.
The A-10 isn't opposed to going to 16 teams, but then would have to have a third member, possibly out of the CAA, to get to that number if Charlotte were to leave. The A-10 could simply pluck one of the two CAA Virginia schools to replace the 49ers but it's hard to say if they would be split apart.
Regardless, the A-10 pulled off a coup in basketball realignment that is akin at its level to the ACC getting Syracuse and Pitt from the Big East in the power six. The A-10 had to replace a legitimate tradition-rich basketball school in Temple with a comparable school and did so with Butler.
The A-10 is expected to stay at 14 teams in one division. But if the A-10 were to go to 16 teams then it would explore a two-divisional format. But the source said there hasn't been any discussion of moving to divisions.
Butler's departure will crush the Horizon League. The Horizon League is left with lower-profile Midwestern schools in Cleveland State, Loyola (Ill.), Detroit, Illinois-Chicago, Green Bay, Milwaukee, Valparaiso, Wright State and Youngstown State.