Thiem had been the only player to have beaten Nadal on clay in the last two years - in Rome last year and Madrid this year - but at no stage did he pose a real threat to Nadal who went from strength to strength as the match went on.
Since the start of the Open era in 1968, the players from the US were the dominant figures in men's tennis at Grand Slam tournaments, winning 51 titles and leaving Sweden, Spain and Switzerland far behind.
Thiem, who had already recorded his best result at Grand Slams with a final appearance in Paris, will rise to No. 7 on world rankings.
The 32-year-old Spaniard is also vying for his 17th major trophy, which would move him within three of Roger Federer.
Despite the loss, the 24-year-old Austrian said it had been a "great two weeks" for him. Thiem clawed his way back to deuce, but Nadal claimed victory on his fifth match point after the Austrian sent a backhand return long.
"It's awesome, bravo. I saw you win this in 2005 when I was just 11 years old".
Nadal's introduction included a year-by-year accounting of every time he'd already won the title at Roland Garros.
Two breaks later Nadal was in a familiar position, serving for the title, and when another Thiem forehand dropped long, his arms were raised in celebration, yet again.
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Thiem's serving and shot-making under pressure were among his principle weaknesses in losing the first set . In contrast, there are eight women now younger than 28 who have been Grand Slam singles finalists.
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Nadal is now the fourth man in the Open Era to win three or more Grand Slams after turning 30, having won his third since turning 30.
Thiem is in trouble as Nadal breaks to take a 2-0 lead in the second set.
"And the strength that he has and the mentally, everything is ideal, works flawless for him playing in clay".
Another feature was his ridiculously slow play when serving, nearly always over the 25-second limit and sometimes going well into the thirties.
Thiem was considered the stiffest test for Nadal at the French Open and understandably so.
The top-seeded Nadal enters Sunday with a 10-0 record in title matches at Roland Garros and an overall career mark of 85-2 at the clay-court Grand Slam tournament.
Dominic Thiem couldn't get near the veteran on the day, despite the fact the Austrian had been the only player to defeat Nadal on Clay during this current season.
The forecast says there is a chance of rain.