It was the second consecutive week that Nadal had won a tournament for the 10th time. He had become the first men's tennis player in the Open era to win the same title 10 times at the Monte Carlo Masters last Sunday.
"It means a lot to me to win 10 titles here in Barcelona as well,'' Nadal said. "To win in Barcelona and Monte Carlo gives me a dream start to the clay season.''
It was Nadal's second title of the season and 71st of his career. The fifth-ranked Spaniard had lost his previous three finals, including to Roger Federer in the Australian Open.
Next month he will try to win a 10th French Open title. The last of his 14 Grand Slams was three years ago in Roland Garros.
Nadal broke the ninth-ranked Austrian late in the first set and early in the second, then cruised to close out the match for his 51st career title on clay. He saved the only break point he conceded to Thiem at the "Rafa Nadal" center court.
"It was vital for me to win the first set,'' Nadal said. "It was difficult. I had a few more chances than him, but it was very even.''
The 30-year-old Nadal has won 10 consecutive matches and is 21-1 in his past 22 sets.
Thiem, who had upset top-ranked Andy Murray in the semifinals on Saturday, was trying to win his second title of the season. He won in Rio de Janeiro in February.
Nadal won five straight titles in Barcelona from 2005-09, then three consecutive from 2011-13, and again in 2016.
He improved to 53-3 at the ATP World Tour 500 event.