With her victory on Saturday, Stephens became only the fifth unseeded player to win a women's Grand Slam title.
In a match between close friends that lasted just over one hour, Stephens made only six unforced errors in the match, compared to 30 for Keys.
"Maddy is one of my best friends, if not my best friend on tour, and I told her I would not have wanted to play anybody else," Stephens said. "It's a special moment. I said to her that I wished it could have been a draw."
Keys, seeded 15th, and 83rd-ranked Stephens had both fought off injuries to reach the title showdown.
"I had surgery on January 23 and if someone had told me then that I would be the US Open champion, I would have said that would be impossible," said Stephens.
Stephens will jump to 17th in Monday's world rankings while Keys, who took home $1.825 million (€1.5 million) as runner-up, will rise to 12th.
Neither Keys, 22, nor Stephens, 24, had ever reached a Grand Slam final, only the seventh time in the Open Era (since 1967) that two first-timers met for a women's Grand Slam title.
jbh/sms (Reuters, AFP)