The five-time All-Star selection agreed to a four-year, $97 million deal with the Sixers, according to ESPN. He could make up to $109 million with championship bonuses.
Horford, 33, will leave the Boston Celtics after three seasons. He declined his $30 million player option for next season to become a free agent. The 76ers plan to put the big man at power forward alongside center Joel Embiid, according to ESPN.
Horford began his NBA career with the Atlanta Hawks, spending the first nine years of his career in Atlanta. He signed with the Celtics in 2016 and averaged 13.6 points, 6.7 rebounds and 4.2 assists per game with Boston this season.
NBA free-agent contracts can't be officially signed until July 6.