James, who also picked up the regular season MVP honour for the third time in his career, led the Heat to a 4-1 win over the Oklahoma City Thunder in the best-of-seven championship series that was capped off by a 121-106 victory.
The dominant player throughout the series, James scored 26 points, pulled down 11 rebounds and doled out a game-high 13 assists in a brilliant all-round effort in the finale.
"This means everything," the 27-year-old James said after being presented with the MVP trophy by Celtics great Bill Russell.
"It's a dream come true."
James, who averaged 28.6 points, 10.2 rebounds and 7.4 assists in the series, won his first NBA ring on his third trip to the finals.
His Cleveland Cavaliers were swept in four games in 2007 by the San Antonio Spurs, and the Heat lost to the Dallas Mavericks in six games last season.
"Losing the finals last year put me back in place, it humbled me a lot," said James.
"Today is the happiest day of my life."
Previous Finals MVPs:
2011 - Dirk Nowitzki, Dallas Mavericks
2010 - Kobe Bryant, Los Angeles Lakers
2009 - Kobe Bryant, Los Angeles Lakers
2008 - Paul Pierce, Boston Celtics
2007 - Tony Parker, San Antonio Spurs
2006 - Dwyane Wade, Miami Heat
2005 - Tim Duncan, San Antonio Spurs
2004 - Chauncey Billups, Detroit Pistons
2003 - Tim Duncan, San Antonio Spurs
2002 - Shaquille O'Neal, Los Angeles Lakers
2001 - Shaquille O'Neal, Los Angeles Lakers
2000 - Shaquille O'Neal, Los Angeles Lakers
1999 - Tim Duncan, San Antonio Spurs
1998 - Michael Jordan, Chicago
1997 - Michael Jordan, Chicago
1996 - Michael Jordan, Chicago
1995 - Hakeem Olajuwon, Houston
1994 - Hakeem Olajuwon, Houston
1993 - Michael Jordan, Chicago
1992 - Michael Jordan, Chicago
1991 - Michael Jordan, Chicago
1990 - Isiah Thomas, Detroit Pistons
1989 - Joe Dumars, Detroit Pistons
1988 - James Worthy, Los Angeles Lakers
1987 - Magic Johnson, Los Angeles Lakers
1986 - Larry Bird, Boston Celtics
1985 - Kareem Abdul-Jabbar, Los Angeles Lakers
1984 - Larry Bird, Boston Celtics
1983 - Moses Malone, Philadelphia 76ers
1982 - Magic Johnson, Los Angeles Lakers
1981 - Cedric Maxwell, Boston Celtics
1980 - Magic Johnson, Los Angeles Lakers
1979 - Dennis Johnson, Seattle SuperSonics
1978 - Wes Unseld, Washington Bullets
1977 - Bill Walton, Portland Trail Blazers
1976 - Jo Jo White, Boston Celtics
1975 - Rick Barry, Golden State Warriors
1974 - John Havlicek, Boston Celtics
1973 - Willis Reed, New York Knicks
1972 - Wilt Chamberlain, Los Angeles Lakers
1971 - Kareem Abdul-Jabbar, Milwaukee Bucks
1970 - Willis Reed, New York Knicks
1969 - Jerry West, Los Angeles Lakers