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Nadal moved on to the same number of titles as Monaco's compatriot Guillermo Vilas.
The start of the match was delayed because of rain and there was a further hold-up at 1-1 in the opening set.
But once play got started, the 28-year-old swept to an easy victory.
"I had the hope of winning here and I prepared very well for this so it's objective achieved," the nine-time French Open champion said.
It was his first final since winning at Roland Garros last June, having been hampered by a wrist injury and appendicitis in the second half of 2014, and was his 65th career title.
Nadal got the first break in the seventh game when the Argentine netted a service return.
He went on to break serve three more times for a comfortable victory in one hour and 26 minutes.
Nadal moved up to third in the world rankings after the victory, overtaking Britain's Andy Murray, who is now fifth, with Japan's Kei Nishikori fourth.
Spain's Rafael Nadal won a record-equalling 46th career clay-court title with a 6-4 6-1 victory over home favourite Juan Monaco in the Argentina Open in Buenos Aires.