The world number one required less than three hours to shake off his Austrian opponent, prevailing 6-4 6-3 6-2 in Paris.
Nadal now has 17 Grand Slam titles to his name, leaving him just three short of Roger Federer's all-time record in the men's game.
A tight opening set saw Thiem hit back in the third game after losing his break of serve in the second, but Nadal was soon at his athletic best and was able to grab a big foothold in the match.
Thiem, competing in his first major final, was second best in the next set as he was edged out in a gruelling second game for the only break of serve.
After racing 5-2 in front in the final set, the Spaniard was frustrated somewhat as it took four match points before finally clinching his latest tournament victory.
Only Margaret Court with 11 Australian Open titles has won the same Grand Slam more often than Nadal has at the French Open, doing so between 1960 and 1973.