Massa received 36.5% of the votes and Milei gained 30.09% after 95.54% of the votes were counted. Pre-election polls predicted a win by Milei.
The stronger than expected performance by the Peronist coalition candidate comes as the country faces triple digit inflation and increasing poverty.
Neither incumbent President Alberto Fernandez nor his deputy and two-time president Cristina Fernandez de Kirchner decided to run in this year's elections.