The 25-year-old American collected a total of 8,790 points for a clear win by 150 points, ahead of Olympic bronze winner Leonel Suarez of Cuba.
Russia's Aleksandr Pogorelov, who had held a world leading score prior to the tournament, socred 8,528 points, his personal best score, to win the bronze.
"It just shows what faith can do," Hardee said, referring to his disappointment in Beijing.
He said that his failure in Beijing had given him extra motivation to come back.
"I didn't have fun last year and tried to make it to the point I have fun this year. I'm incredibly happy. Nobody can be more happy than I am now.
"I recorded a personal best after the first day and based on those results, we were confident the second would be better," he said.
Suarez said, "I was not happy about my first day. But at the end, everything was very good for me. I missed only 14 points to anew Cuban record."
He believed that he will be able to win the title in the future.
"I am very young, so I can try to win the world title in two years. After my bronze last year in the Olympics I now improved to silver," Suarez said.
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