The 19-year-old ninth seed belted 35 winners to become the first Russian woman to win the U.S. Open and the third to capture a grand slam in 2004, the first time since 1979 three women from the same country have won the top prizes in tennis.
Anastasia Myskina picked up the French Open in June and Maria Sharapova lifted the Wimbledon crown the following month.
"Russia is just too powerful a country," Kuznetsova joked to the crowd in her courtside interview after sealing victory in one hour 14 minutes with a service winner.
Kuznetsova said she had received support from 18-times Grand Slam singles champion Martina Navratilova, her former doubles partner, before the match.
"I would like to thank Martina, a legend," she said. "I took your words on to court with me, you won in your first grand slam final and I did it too today."
Sixth seed Dementieva, who lost to Myskina in the Roland Garros final, began strongly, winning the first four points to break Kuznetsova to love.
But Kuznetsova soon found her range, taking advantage of her opponent's weak serve to break back in the following game and then again in the sixth game on the way to the opening set.
Dementieva, wearing a heavy strapping on her left thigh, began to open up with the powerful forehand that caused so much damage against Jennifer Capriati in Friday's semi-final, breaking Kuznetsova to move 2-0 ahead in the second set.
MISSED FOREHAND
But she could not consolidate and a missed forehand allowed Kuznetsova to break straight back.
The French Open runner-up seemed to take a grip of the set again when she broke to lead 4-2. But her younger opponent kept pounding huge forehands and the 22-year-old's serving demons returned when two double-faults gave Kuznetsova the break back.
Dementieva saved another break point at 4-4 but two games later a fourth double fault handed Kuznetsova the chance to serve for the match and she completed victory on her second match point.
"I wasn't able to play my best tennis today," Dementieva said, referring to her thigh injury. "But Svetlana played a great game and deserved to win.
"It's been a great tournament for me. I played some good tennis and I am not that disappointed. She just played better than me.
"I'm very proud that two Russians were in the final. It's a miracle to me that three Russian girls have won grand slams in one year."
After the match, played on the third anniversary of the devastating attacks on the U.S. which killed nearly 3,000 people, Kuznetsova and Dementieva paid tribute to the dead and also those who perished in the recent hostage siege in Beslan, southern Russia.
"I want to dedicate this trophy to all the people who died (in both incidents)," Kuznetsova said.
"It was difficult to go on court and enjoy tennis when something like this happened in your country," Dementieva said.
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