Nadal garnered his eighth career win at Barcelona with a 6-4, 6-3 decision over fellow Spaniard Nicolas Almagro.
It was his 39th straight win at the event, which he skipped in 2010.
Nadal went down 3-0 in the first set but then dominated, improving to 10-0 all-time against Almagro.
Since his February return from a seven-month layoff due to a knee injury, Nadal has gone 26-2, including 21-2 on clay, claiming earlier titles at Sao Paolo, Acapulco and Indian Wells.
The Barcelona victory came one week after he was surprised by top-ranked Novak Djokovic in Monte Carlo, where his string of eight straight championships was broken.
Nadal improved to 39-6 all-time in clay-court finals.
Almagro, meanwhile, fell to 12-8 in finals, all of which have been on clay.