The King of Clay confirmed that he is still the best player ever to grace the Parisian clay, easing past the Austrian 6-4, 6-3, 6-2 on Court Philippe Chatrier.
"It's really just incredible".
To Thiem's credit, he played for broke in Sunday's final, going for winners.
A left hand cramp in the third and final set proved to be no real obstacle for Nadal, the Spaniard showing he was approaching his best form ahead of the grass court season.
Nadal called it "not even a dream" to gather 11 titles in Paris, saying it was "impossible to think of something like this".
"But if you win a Grand Slam tournament 11 times, one single one, then this is just very exceptional and wonderful".
He also won Wimbledon for a second time in 2010 before claiming his first US Open title, becoming the seventh man to win all four Grand Slams.
"What you did and what you are doing", Thiem told Nadal during the trophy ceremony, "is one of the most outstanding things that any athlete is doing in any sport".
In 2007, he became the first man since Bjorn Borg in 1980 to win a hat-trick of Roland Garros titles.
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With a celebrity audience - including actors Hugh Grant and Tim Roth as well as French stars Marion Cotillard and Jean Dujardin - watching, Nadal flew out of the blocks.
Nadal's introduction included a year-by-year accounting of every time he'd already won the title at Roland Garros.
"And in general. game-wise it was a good game".
He saved another two in the 11-minute sixth game, a performance cheered by the crowd who briefly became more animated when glum-looking French football icon Zinedine Zidane was spotted in the stadium.
After winning just one point in the first two games, Thiem dragged himself back into the contest, only to throw away the opening set with an erratic 10th game as he was broken to love.
A wild, misguided forehand sealed his fate, one of 18 unforced errors to Nadal's 12 in the set.
Theim lost his maiden Grand Slam final in straight sets against Nadal at Roland Garros. Thiem stayed with Nadal in the early going on a steamy afternoon.
He also let four match points go at 5-2 but he would not be denied as Thiem went long with a backhand on the fifth.
He led by two sets plus a break early in the third, when the middle finger on his racket-wielding left hand was cramping so badly he couldn't straighten it. Nadal is a huge Madrid fan, and is often seen at games.
He still won the game anyway before summoning the doctor and trainer courtside.