Despite being just 19 years old, Walcott produced a second throw of 84.58m, which was enough to secure top spot ahead of his rivals.
"I just went out there to relax and enjoy it and it worked for me," he told reporters.
"It means everything to me. I just train my hardest and try to enjoy every time I come out."
Walcott is only the second non-European to win the title in 100 years of the Olympic Games.