Conceding defeat before official results came out, Massa said he has congratulated Milei, who will be the country's president for the next four years, Xinhua reported.
According to official data released by the government, with 86.59 percent of the votes counted, Milei obtained 55.95 percent, far ahead of Massa's 44.04 percent.
Sunday's poll took place with an estimated turnout of 76.39 percent.
Milei will succeed current President Alberto Fernandez and is set to take office on Dec. 10 for a four-year term.
AMK/PR