Since returning from a seven-month absence with a left knee injury in February, the world number five has reached the final at all six events he has played, his Barcelona triumph adding to the victories in Sao Paulo, Acapulco and Indian Wells.
"I'm very happy. It has been an important week for all the Spanish players here and especially for me," Nadal told Spanish television.
"It is an enormous honour for me to win here again."
Having lost his Monte Carlo Masters crown to world number one Novak Djokovic in last Sunday's final, Nadal was on the back foot again against Almagro in a shaky opening.
The fourth seed made a whirlwind start, breaking Nadal's first two service games in cloudy, drizzly conditions on the clay of the Real Club de Tenis.
Nadal battled back with three breaks of serve to take the first set, lifting the rain-soaked crowd with one particular rally which he eventually won after having played a shot backwards between his legs.
The French Open champion broke again early in the second set and steadily pulled away to secure a 10th successive victory over his compatriot and a 39th consecutive win in Barcelona.
"Nico started really strongly, and it was very difficult at the start. The conditions were tough for both of us," second seed Nadal said.
"It didn't start very well for me but luckily I was able to get the very important break at 3-0 down.
"I'm pleased for Nico for his great week. He's having a great season and I wish him all the best for the future."
Almagro was able to make light of his defeat when asked about how he started like a motorbike.
He laughed and said: "Yes, well the motorbike got stuck in the rain.
"Rafa has shown once again why he is the best player in history on this surface. He is one of the greats on the tennis circuit and as Spaniards we are all very proud of him.
"I got a good start but the surface was tricky, though that cannot serve as an excuse, Rafa was the fair winner.
"I'll have another go next year, and try to win it then if Rafa lets me."