Spoelstra hinted two weeks ago that he would remain loyal to the Heat regarding any decisions he had to make in his first assignment as an All-Star coach.
Bosh will replace injured guard Rajon Rondo, who is recovering from season-ending knee surgery.
All-Star starters are chosen through fan voting.
There was a possibility that Spoelstra would have taken a conventional approach by replacing Rondo with another guard, with Kyrie Irving being a popular alternative.
But Spoelstra announced his decision to the Heat's team-run website Friday about an hour before he was set to meet with reporters for the scheduled All-Star media availability for players and coaches.