Nadal's latest triumph in Paris sees him join Margaret Court, who dominated the Australian Open in the 1960s and 70s, as the only player to win 11 singles titles at one grand slam.
Roland-Garros hosted a final that told two stories, with Nadal easing to victory after surrendering only one set all tournament long, while Thiem looked noticeably short on stamina, particularly in the later stages of Sunday's decider.
The 32-year-old World No 1 now has 17 Grand Slam titles, just three behind great rival Roger Federer.
The king of clay has his first major of 2018 and will be eager to add a second when the Grand Slam calendar turns to Wimbledon in July, although it's been eight years since he last made the final at SW19.
Up in the stands, Nadal's uncle Toni, his former coach, was nervous, too, "because I thought maybe we can have a problem", he said. He pounded huge serves that topped 135 miles per hour - about 25 miles per hour better than Nadal's fastest - and translated into seven aces but also had five double faults.
Nadal came in looking slightly more vulnerable than he did past year, in which he lost only 35 games in claiming the title. Whether it was at the Masters tournaments in Monte Carlo, Madrid, or Rome-whether it was his home tournament in Barcelona-Nadal nearly never lost on the Spaniard.
"I still remember when you won here the first time in 2005".
He's the only player to beat Nadal on clay in the past two years - in Rome last year and in Madrid last month.
Rafael Nadal is the French Open 2018 victor.
- Title number nine in his ninth final for Nadal and his 14th Grand Slam success on an afternoon where temperatures nudged 30 degrees.
Injuries pockmarked Nadal's early season and spawned doubt as he prepared to start the clay-court season that has traditionally served as a long taxi-way for Paris. The Austrian followed that point up with three consecutive errors, and the first set went to the Spaniard.
Up a break at 2-1 in the third set, Nadal stopped serving after a fault because he couldn't straighten his left middle finger.
Just like that, Nadal was off on a five-game burst to lead 3-0 in the second set. Thiem broke the Spaniard's serve in the third game of the match, nullifying Nadal's early break but earned only one more break point - which was saved by Nadal. Throughout his 14 years competing here, Nadal has only lost two matches. After guzzling water during his visit from the doctor, Nadal was back to playing his unmistakable brand of almost unbeatable clay-court tennis.