The top-seeded Spaniard hadn't reached a final since winning at Roland Garros last year.
It was Nadal's 46th title on clay, moving him within three of the record set by Argentine Guillermo Vilas when he played in the 1970s and early 1980s.
Nadal broke Monaco's serve late in the first set to take a 1-0 lead against the 60th-ranked Argentine, then dominated the second to close out the match in 1 hour, 26 minutes.
Rain in Buenos Aires delayed the start of the match and caused its interruption at 1-1 in the first set.
Nadal, who had to deal with injuries in recent months, now has 65 singles titles.