Haslem, 39, announced his return in an Instagram post Tuesday afternoon. He joined the Miami Heat in 2003 after a year in France.
Haslem will become the first Heat player to play in three separate decades if he appears in a game in 2020.
A three-time NBA champion, Haslem's biggest contributions have been off the court in recent years. He's played only 50 games the last three seasons as the Heat use him as a clubhouse leader and mentor.
Haslem averaged 3.6 points and 2.7 rebounds in 10 games (one start) last season.
Haslem has averaged 7.6 points and 6.7 rebounds in 854 career games. He's added 5.7 points and 5.6 rebounds in 147 career playoff games.