It was the third time in four years that "King" James has won the game's most prestigious individual award, elevating him alongside the sport's biggest names.
Only seven other players - Kareem Abdul-Jabbar, Michael Jordan, Bill Russell, Wilt Chamberlain, Larry Bird, Magic Johnson and Moses Malone - have won the trophy at least three times.
Abdul-Jabbar holds the record for six MVP wins, one ahead of Jordan and Russell but James, still just 27 years old, has plenty of time on his side.
"I never dreamt about being an NBA Most Valuable Player," an emotional James said in a presentation in Miami broadcast live on national television.
"I never dreamt about doing the things that I do now at a high level.
"Now that it's in me now, and it's happening, it's overwhelming."
James, who twice won the award with the Cleveland Cavaliers before moving to Miami last season, averaged 27.1 points, 7.9 rebounds and 6.2 assists during the 2011-12 regular season, which was shortened to 66 games because of a labor dispute.
He totalled 1,074 points, including 85 first-place votes, from a panel of 121 voters that decided the award.
"We do not take LeBron James for granted, not here in this organization," Miami Heat coach Erik Spoelstra said.
"He's as committed as ever... and we all respond to his energy on the court."
Oklahoma City's Kevin Durant, the leading scorer for the third season in a row, finished second in the vote, followed by Chris Paul of the Los Angeles Clippers and Kobe Bryant from the Los Angeles Lakers.
Chicago's Derrick Rose, who won the MVP award last year, finished 11th in the voting after missing more than a third of the season because of injury.
James, despite his individual success, has yet to win an NBA title, but has perhaps his best chance this season with Miami.
"This is very overwhelming to me as an individual award," James said.
"But this is not the award I want, ultimately. I want that championship. That's all that matters to me."
The Heat, who finished runners-up to Dallas last year after James left Cleveland to link up with Dwyane Wade and Chris Bosh in Miami, cruised through the opening round of the playoffs.
They now face the Indiana Pacers in a best-of-seven series, starting in Florida on Sunday, for a place in the Eastern Conference final.
James is sure to feature in the star-studded United States team at this year's London Olympics after being a member of the team that won the gold medal in Beijing four years ago.