Haslem was born in Miami, Florida. His mother, Debra, was Puerto Rican. He attended Wolfson High School in Jacksonville, Florida, then Miami Senior High School in Miami. He helped lead Miami High to state titles in 1997 and 1998 (the last two of three in a row), playing alongside another future NBA player, Steve Blake. The team was coached by current South Carolina head coach Frank Martin.
However, an investigation by the Miami New Times revealed that Haslem, Blake and several other players circumvented residency requirements. The New Times reported that while he claimed to live in Miami, he actually lived in Miramar. His official school address was an efficiency apartment owned by a longtime Miami High booster, which was also a violation of Florida High School Athletic Association (FHSAA) rules. As a result, Miami High was stripped of its 1998 title and ordered to forfeit its entire schedule.
Haslem accepted an athletic scholarship to attend the University of Florida in Gainesville, Florida, where he played for coach Billy Donovan's Florida Gators basketball team from 1998 to 2002, while majoring in leisure service management. As the Gators' starting center for four years, he was part of Donovan's 1998 recruiting class that raised the national prominence of the Florida Gators basketball program, and included future Miami Heat teammate Mike Miller. Fans would yell the letter "U" in unison after his big plays. His sophomore season saw the team advance to the NCAA Men's Division I Basketball Championship game, before falling to the Michigan State Spartans 89–76. The Gators received NCAA tournament invitations four consecutive years during his college career–the first time in the program's history. During his tenure with Florida, Haslem averaged 13.7 points per game and 6.7 rebounds per game. Haslem also ranks third in team history in points scored (1,782) and tenth in rebounds (831). He was inducted into the University of Florida Athletic Hall of Fame as a "Gator Great" in 2012.
After leaving Florida in 2002, his weight ballooned in excess of 300 pounds. Haslem then signed with Chalon-Sur-Saône, a professional team in France. While averaging 16.1 points and 9.4 rebounds per game, Haslem also managed to lose 70 pounds over the course of that year, which resulted in him earning a spot in the NBA summer league.
He was signed by the Miami Heat as an undrafted rookie in 2003. As a rookie, he backed up Brian Grant and provided rebounding and defense. He was named to the NBA All-Rookie Second Team, and played in the rookie challenge during All-Star weekend. He assumed the starting power forward position in 2004 after teammate Lamar Odom was traded as part of a package that brought in superstar center Shaquille O'Neal from the Los Angeles Lakers. He was helpful yet again, being a tough rebounder and developing an effective 15-foot jump-shot. He had been called a perfect complement to O'Neal's style of play, and was critical in the Heat's playoff run to the Eastern Conference Finals against the Detroit Pistons. Following the season his two-year contract was renewed after upping his rookie averages of 7.3 points and 6.3 rebounds to 10.9 points and 9.1 rebounds in his second season.
In 2005–06, he again was the starting power forward. Despite increased pressure from O'Neal battling injuries and the acquisition of three-time All-Star Antoine Walker, Haslem retained the starting spot. On April 22, 2006, in a first-round playoff game against the Chicago Bulls, Haslem was ejected for throwing his mouthpiece at referee Joe Crawford. Haslem had been upset that a foul was not called on a play on the Bulls' end of the court. After the game, he claimed that he had not deliberately aimed at Crawford, and that the mouthpiece's trajectory was an accident. He apologized to fans and to his teammates, and stated that he would also apologize to Crawford when the opportunity arose. The NBA reviewed the incident, and they suspended Haslem for game 2 of the series against the Bulls on April 24, 2006.
Haslem started every playoff game as the Heat defeated the Bulls, Nets and Pistons before reaching the Finals against the Dallas Mavericks. He scored 17 points in the title-clinching game 6.
On November 10, 2006, Haslem scored a career-high 28 points on 10-for-14 shooting and grabbed nine rebounds in a 113–106 win over the Nets. He would continue to start at power forward in the 2006–07 regular season. In the 2007–08 season with Shaquille O'Neal having been traded and Alonzo Mourning going down with an injury, Haslem played the center position and averaged 12 points and 9 rebounds.
In the 2008–09 season, Haslem started at power forward as the Heat reached the playoffs before falling in 7 games to the Atlanta Hawks. The following season, Haslem came off the bench with Quentin Richardson filling in the small forward position and Michael Beasley being named the starting power forward.
Following the Heat's acquisition of LeBron James and Chris Bosh, and the re-signing of Dwyane Wade, there was speculation as to whether Haslem would remain with the team or sign elsewhere. On July 12, 2010, Haslem re-signed with the Heat, even though he would have received nearly $10 million more if he had signed with the Dallas Mavericks or Denver Nuggets. Thirteen games into the season, Haslem was sidelined by a torn ligament in his foot. He would return during the playoffs against the Boston Celtics and had a breakout performance in game 2 of the Eastern Conference Finals against the Chicago Bulls, scoring 13 points in 23 minutes. The Heat would advance to the Finals but would lose in 6 games to the Dallas Mavericks.
In the 2011–12 season Haslem averaged 6 points and 7.3 rebounds in 64 games backing up Chris Bosh. He would play extended minutes when Bosh was hurt during the East semifinals against the Indiana Pacers, and posted double figure scoring games in games 4 and 5. He would be suspended for game 6 following a flagrant foul in the previous game. After defeating Indiana and then defeating the Boston Celtics in 7 games, the franchise would win its second championship against the Oklahoma City Thunder with Haslem and Wade being the only remaining players from the 2006 championship team. Haslem averaged 4.8 points and 6.4 rebounds in the 2012 playoffs.
In the 2012–13 season, Haslem continued to come off the bench for the Heat. On November 21, 2012 in a game against the Milwaukee Bucks, Haslem passed Alonzo Mourning to become the franchise leader in total rebounds with 4,808, breaking Mourning's previous record of 4,807. Mourning was in attendance for the game and applauded Haslem for his milestone. Haslem also became the first undrafted player to lead a franchise in rebounding. During the 2012–13 regular season, Haslem averaged a career-low 18.9 minutes per game, but his shooting percentage from the field rose to 51% after dropping to 42% the previous season. In the 2013 playoffs, Haslem shot 8 of 9 from the field in both Game 3 and Game 5 of the Eastern Conference Finals against the Indiana Pacers. He won his third championship on June 20, 2013, when the Heat defeated the San Antonio Spurs in an intense seven game series.
In the 2013–14 season, Haslem played in a reduced role. The Heat finished with 54 wins and the 2nd seed in the East behind the Indiana Pacers, who won 56 games. Haslem and the Heat defeated the Pacers in the Eastern Conference Finals to advance to their fourth straight Finals in 2014. The Heat lost in the Finals to the San Antonio Spurs in five games.
On June 28, 2014, Haslem decided to opt out of his contract to become a free agent. On July 18, 2014, Haslem signed a two-year, $5.59 million deal with the Heat. On February 20, 2015, Haslem made his first career three-pointer in a 111-87 win over the New York Knicks.
On July 10, 2016, Haslem re-signed with the Heat.
On July 20, 2017, Haslem again re-signed with the Heat. He is currently holding the fourth-longest streak by any active player with only one team in the league.
Haslem appeared in the music video for the song "GDFR" by Flo Rida, as well as the music video for "Bet That" by Trick Daddy, and the music video for Born-N-Raised by DJ Khaled, Pitbull, Trick Daddy, and Rick Ross.
Haslem is married to Faith Rein, a sports broadcaster, whom he dated for 14 years. He met Rein in 1999 while attending the University of Florida, where she was a member of the Florida Gators track and field team. In 2007, they had their first child, a son named Josiah, and they later had a second son named Elijah. On August 29, 2012, Haslem proposed to Rein. A year later, on August 24, 2013, they hosted a wedding in New York City which LeBron James, Dwyane Wade, and other Miami Heat teammates and coaches attended.Most rebounds (5701)
Most defensive rebounds (4101)
Most offensive rebounds (5701)
Most offensive rebounds (230)
3× NBA champion: 2006, 2012, 2013
NBA All-Rookie Second Team: 2004
First undrafted player in NBA history to set franchise rebounding record: 2012