Based on how little opposition Hingis offered, it was a major setback for the former World No. 1, who is making a comeback after a three-year hiatus.
It was the fifth career title for Dementieva, who earned $196,900.
Hingis failed in her quest for a fifth career title at Tokyo. The five-time Grand Slam champion last met -- and lost to -- Dementieva in 2002 at Filderstadt.
She had advanced to the title match on Saturday with a surprise 6-3, 6-1, victory over defending champion and top seed Maria Sharapova in the semifinals.
"I've been playing well for the last five or six weeks," Hingis said. "I guess I just faced one Russian too many."
Both Dementieva and Sharapova are from Russia.
Dementieva, 24, was an Olympic silver medalist at Sydney four years ago.