Nadal beat Dominic Thiem 6-3, 5-7, 6-1, 6-1 in a rematch of last year's final.
That leaves the left-hander two adrift of Switzerland's Roger Federer, who he beat in the semi-finals, and three clear of Serbian world number one Novak Djokovic, whose bid to hold all four majors was ended by Thiem.
"It's just been a insane two weeks", Barty told the crowd.
He leads their head-to-head 8-4. Thiem's coach Nicolas Massu asked tournament officials on Saturday to move the finals to Monday, but had his request denied.
The Austrian did dig deep though and he had the opportunity to break Nadal in the first game of the fourth but the 11-time victor responded in style, producing a vital hold.
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Thiem got the only break of the second set in the final game when Nadal missed a series of shots under pressure from the Austrian player's heavy groundstrokes.
Thiem, bidding to become just Austria's second Roland Garros men's champion after Thomas Muster in 1995, describes facing Nadal in Paris as "the ultimate challenge".
"I think it's really important that I go into the match with the belief to win", he said. It was six weeks ago.
Factor in the two-day contest with Djokovic, which lasted four hours and 13 minutes overall, and Thiem's chances of preventing Nadal collecting a 12th title this afternoon appear annoyingly slim. "So of course I try to do similar even though it's way tougher to play him here", he said.
Dominic Thiem celebrates after winning against Novak Djokovic on Saturday.
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"I can not explain my feelings. I'm sure Rafa when he woke up in the morning, he was like, 'Oh yay, that's going to be a good day in the office'". As it transpired the tournament was hasty in doing so, because the rain did not come and they could have come back out and played on for around an hour. "So I'm ready to leave all or everything what I have out on the court tomorrow".
Some of the statistics back up his conservative approach.
More than an hour after the women's final should have started on another stormy day on Court Philippe Chatrier, fourth seed Thiem thumped a forehand victor past Djokovic to convert his third match point.
On break points converted, the Spaniard has a 37/70 conversion rate (53%) with Thiem making 34 of 71 (48%), less than half his opportunities.
"It's a dream to win again, an incredible moment".
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