Macri said on Twitter that he chose Peronist Sen. Miguel Angel Pichetto because Argentina will "need to build agreements."
The conservative president is seeking re-election amid a recession, a devaluation of the currency and an inflation rate that at around 50% is one of the world's highest. The economic crisis has fueled labor protests against Macri and dented his approval ratings.
His main challenge will come from leftist former President Cristina Fernández, who surprised Argentines last month when she announced that she would try to return to power by running as vice president on the ticket with a former chief of staff, Alberto Fernández.
Pichetto was allied with Fernández during her 2007-2015 administration, but has since broken with her and headed a more moderate opposition faction within the always fractious Peronist party.
"I'm not going back to the past," Pichetto told local media earlier Tuesday. "A victory by Alberto Fernández and Cristina Fernández is a return to the past."
Cristina Fernández is known for her populism and unorthodox economic policies. While her supporters believe she led Argentina out of an economic crisis, others blame her for creating conditions that led to the country's current turmoil.
Macri is credited with returning Argentina to global credit markets for the first time since its worst economic crisis in 2001-2002, but he has failed on promises to curb inflation and end poverty. After a sharp depreciation of Argentina's currency, he was also forced to seek a record $56 billion bailout loan from the International Monetary Fund to try to tame the crisis.