Thiem kept with Nadal and threatened to cause a major upset but the Spaniard elevated his level to seal a 6-3 5-7 6-1 6-1 win.
It is the second year in a row that Nadal has beaten Thiem in the French Open final.
But in his post-match speech, Nadal hailed his opponent and claimed he would eventually win the title in the future.
“First thing I want to say is congrats to Dominic. I feel sorry – he deserves it,” Nadal said.
“He has absolute intensity and passion for this sport. He’s such a hard worker. I wish him all the best for the future.
“I can’t explain what I’ve achieved and how I feel. It’s a dream.
“To play for the first time in 2005 – never thought in 2019 I’d still be here. It’s an incredible moment and very special for me.”
Former British number one Greg Rusedski said on BBC Radio 5 Live: “Nadal is enjoying this moment more than ever.
“Thirty-three years young and he just gets better like a fine bottle of wine.”
ALL-TIME GRAND SLAM TITLES
Roger Federer – 20
Rafael Nadal – 18
Novak Djokovic – 15
Pete Sampras – 14
Roy Emerson – 12
Rod Laver, Bjorn Borg – 11
Bill Tilden – 10
Fred Perry, Ken Rosewall, Jimmy Connors, Ivan Lendl, Andre Agassi – 9