Dominating the world's No. 1 player with astounding ease, Nadal swept six consecutive games early in the match and swept the final nine games to win 6-1, 6-3, 6-0.
It was Federer's worst loss in his 173 Grand Slam matches, and the most lopsided men's final at Roland Garros since 1977.
So thorough was the thumping that during the trophy ceremony, Nadal was moved to apologize.
"Roger, I'm sorry for the final," Nadal said.
It was merely another in a series of dominating victories by the Spaniard, who lost only 41 games in seven rounds.
"I've hoped I could have done better today than four games," Federer said. "But Rafael was very strong this year."
The No. 2-ranked Nadal became the second man to win four consecutive French Open titles. Bjorn Borg did it in 1978-81.
"Winning four times in a row is incredible," Nadal said. "The comparison with Borg is always very nice, especially because he's much better than me."
Nadal improved to 28-0 at Roland Garros, where he has won 83 of 90 sets. Only six-time champion Borg won more French Open men's titles. And Nadal became the first man since Borg in 1980 to win the tournament without dropping set.
"He dominated the tournament like never before, like Bjorn," Federer said.
For the fourth consecutive year, Federer arrived in Paris seeking to become the sixth man to win all four major titles. Each time he has lost to Nadal -- in the semifinals in 2005, and in the final each of the past three years.
Federer conceded the latest defeat shakes his belief he can win Roland Garros.
"After a loss like this you, don't want to play Rafa again tomorrow, that's for sure," Federer said.
"Roger's going to be back," Borg said, adding with a laugh, "and so will Rafa."
Borg watched from the front row as fans rooted in vain for a competitive contest, chanting "Ro-ger! Ro-ger!" between games. But Nadal earned their cheers, too. He won 24 of 27 points to take a 2-0 lead in the second set as a desperate Federer tried everything to reverse the tide.
But when Federer played serve-and-volley on a second serve, Nadal lunged to hit a lob into the corner for a winner. When Federer tried to chip and charge off a return, Nadal passed him with a backhand.
And when Federer settled for playing from the baseline, he had little chance. It's tough to hit shots where the relentless Nadal can't reach them, and Federer probably tried too hard, with uncharacteristic errors flying from his racket.
"When you really cannot play your game, and he can play exactly what he wants from the baseline, you end up with scores like this sometimes," Federer said. "He hardly made unforced errors, and when he's on the attack, he's lethal."
Federer faced a break point in 10 of his 11 service games, and he held only three times. Nadal was particularly ruthless returning second serves, and he won 19 of those 24 points.
Federer did gain a toehold in the second set when he won consecutive games for the only time for 2-all, but he lost serve again at 3-4. On break point he hit a deep volley that would win most rallies, but Nadal ripped it for a winner.
"When I was playing, I didn't believe the match," Nadal said. "I feel the match has to be closer. ... I played almost perfect."
For Federer, the worst came at the end -- a 6-0 set loss. The last time that happened to him was on a slightly smaller stage: the first round in 1999 at Queen's against Byron Black.
On match point, Federer sailed a forehand long, and Nadal raised his arms in triumph. Nadal improved to 9-1 against Federer on clay, with the only loss coming in the 2007 Hamburg final.
Other statistics that underscore Nadal's dominance on the surface: He's 22-1 in clay-court finals, and 41-0 in best-of-five matches on clay.
And he seems to be getting better.
"He was really on fire today," Federer said. "It's the strongest Rafa that I've ever seen."
Federer fell to 12-3 in major finals, with all of the losses to Nadal. They've met six times in Grand Slam events, and Nadal leads 4-2.
Federer's far from alone in his French Open frustration. The list of Grand Slam champions who never won the Roland Garros title includes Pete Sampras, John McEnroe, Jimmy Connors, Boris Becker and Stefan Edberg.
"To lose the way I did today, it's obviously hard and it's a rough loss, but it's OK," Federer said. "I'll move on from here, and I'll try again next year."