The Ohio prep star is expected to be the top overall pick.
"I'm looking forward to the challenge," James said at a news conference at St. Vincent-St. Mary High School. "It's been a longtime dream for me to play in the NBA. There's no better time for me because I know how hot I am."
James is so hot his sneaker deal with Adidas, Nike or Reebok could net a price tag of nearly $50 million.
Wearing No. 23 in honor of Michael Jordan, which might be a good sign for Nike, James averaged 30.4 points, 9.7 rebounds, 4.9 assists and 2.9 steals this past season.
The 6-8, 240-pound James made his announcement one day after Carmelo Anthony left Syracuse on the heels of delivering the school a national championship in his freshman year.