The Spaniard has made the ideal start to the clay-court season and will aim for a 10th French Open next month after also claiming his 10th title at the Monte Carlo Masters last week.
The 14-time Grand Slam champion met some resistance in the beginning from Thiem, who surprised world No. 1 Andy Murray in Saturday's semifinals.
He threatened to repeat that upset when he served for the match in the third set, but Murray battled back to edge out the world number 19 by 2-6, 6-4, 7-6 (7/4) after three hours on court.
But top seed Pouille was a step up in class on clay from his previous opponents and the Frenchman eased to his second ATP Tour title with a 6-3 6-1 win in just an hour and four minutes.
Thiem won the Rio de Janeiro title this year.
Despite Murray threatening a fightback after taking the second set to take the match to a final-set decider, Austrain Thiem held his nerve to record a 6-2, 3-6, 6-4 victory on clay.
Thiem, a French Open semifinalist last spring and a semifinal victor over world No. 1 Andy Murray on Saturday, threw everything he had at Nadal in the first set.
The world number nine even had his only break point of the match in the first game.
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The duo have met each other thrice in the past, with Nadal now leading 2-1 with their last meeting coming in the Monte-Carlo Masters in 2016.
Rafael Nadal traditionally jumps into a swimming pool when he wins in Barcelona and on Sunday, he didn't disappoint.
But the Scot had 0-30 and 15-40 in the next game and made the most of the chance to break straight back Murray served to stay in match at 4-5 but when he went way long with a smash to hand Thiem a match point the Austrian closed it out.
The third-seeded Nadal broke the fourth-seeded Austrian late in the first set and early in the second before cruising the rest of the way for his 51st career title on clay. I'm not used to playing two of the greatest players in two days.
It's not easy winning on European clay but Rafael Nadal makes it look so.
"He was hitting to my backhand and I couldn't really come out of it", said Thiem. "To play here in front of my crowd in my club and win for the tenth time - it's impossible to dream about it".
"If I'm going to lose to anyone, it should be against someone who is a good player and a good person, which Lucas is".
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