Allen told the Heat on Friday night that he has decided to leave the Boston Celtics and join up with the reigning NBA champions.
Heat owner Micky Arison tweeted the news just after 9:30 p.m., writing, "Its 2:30am in London and I was just woken up with great news. Welcome to the family #20!!"
Allen's agent, James Tanner, confirmed the decision to The Associated Press not long after Arison's tweet. Allen visited Miami Thursday, dining with Heat president Pat Riley, coach Erik Spoelstra, team executive Alonzo Mourning and others. Allen left left Friday, making a decision shortly thereafter.
Financial terms have not yet been disclosed, but Miami likely offered Allen the mini midlevel exemption, worth $9.1 million over three seasons. Shane Battier received a similar deal from the Heat last winter. The Celtics had reportedly offered Allen a two-year deal worth $12 million.
Allen has made more three-pointers than anyone in NBA history. He is a career 40% three-point shooter, and hit 45% of his shots from downtown in 2011-12, the best season of his career percentage-wise.
Allen likely will not start for Miami, but join the Heat's deep stable of wing players who can be used almost interchangeably alongside LeBron James, Dwyane Wade and Chris Bosh. Boston was expected to play him off the bench as well, though.
Allen will be a luxury to have coming off the bench. With Battier, James Jones, and Mike Miller (barring an early retirement), Allen completes a formidable rotation of three-point shooters who can take advantage of LeBron James' blossoming post game.
The NBA Finals provided a taste of the Heat's future, as Battier and Miller feasted on open looks from three-point range. Allen will get plenty of chances to do what he does best in Miami. The Heat, already a co-favorite to win the 2013 NBA title, have solidified its bench on the cheap for the second season in a row.