In a letter to Cavaliers fans, the team's majority owner Dan Gilbert said he was disappointed that James opted to join fellow All-Stars Dwyane Wade and Chris Bosh in Miami and felt the nationally televised announcement was inappropriate.
"If you thought we were motivated before tonight to bring the hardware to Cleveland, I can tell you that this shameful display of selfishness and betrayal by one of our very own has shifted our 'motivation' to previously unknown and previously never experienced levels," Gilbert wrote.
James opted to deliver his decision during an hour-long program on cable network Thursday after weeks of speculation on whether he would stick with his hometown Cavaliers or jump to one of the other five teams who courted his services.
His decision dealt a major blow to a city that has not celebrated a professional sports championship since 1964 and has developed a widely known reputation for falling on tough times in key games.
Without James on the payroll, the Cavaliers can now look to bolster their roster by signing players from what is regarded as one of the deepest pools of free agents.
"I promise you that our energy, focus, capital, knowledge and experience will be directed at one thing and one thing only: delivering you the championship you have long deserved and is long overdue," wrote Gilbert.
"We are thrilled that LeBron James and Chris Bosh have decided to come to Miami to join forces with our truly great player, Dwyane Wade," Heat coach Pat Riley said in a statement.
"We are looking forward to the opportunity of building something that our fans in Miami will be proud of for a long, long time. The journey is just beginning."
(Writing by Frank Pingue in Toronto; Editing by Steve Ginsburg)
