Horford announced his decision on his Twitter account.
"Celtic Pride!!!!!!" he tweeted, followed by 18 green shamrocks.
Horford, a four-time All-Star, spent his first nine seasons with the Atlanta Hawks. He has career averages of 14.3 points and 7.9 rebounds and averaged 15.2 points and 7.3 rebounds last season.
Horford was part of a 60-win Atlanta team two seasons ago and is seen as the type of player who can help the Celtics rise up to perhaps challenge the Cleveland Cavaliers as the top team in the Eastern Conference.
Horford continued negotiating with the Hawks into Saturday night before picking Boston.