Schwartzman never gave the world number 137 a chance and stormed to the title without dropping a set in his four matches this week.
Sumit Nagal started strongly against Albert Ramos-Vinolas at the Argentina Open but errors crept in in the second and third set to lose despite a spirited display.
The world number 29, seeded third in Buenos Aires, lost 4-6, 6-3, 6-1 against the 22-year-old world number 137 Francisco Cerundolo.
Argentina Open: Italy's Marco Cecchinato trounces home favourite Diego Schwartzman to claim third ATP title
Italian Marco Cecchinato won the Argentina Open on Sunday with a clinical 6-1, 6-2 demolition of hometown favourite Diego Schwartzman.
Argentina Open: Dominic Thiem coasts past Aljaz Bedene to win second Buenos Aires title in three years
Dominic Thiem defeated Aljaz Bedene 6-2, 6-4 in one hour and 31 minutes to extend his perfect record at the tournament, having captured the trophy also in 2016.
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.