National deputy Héctor Olivares has died, three days after being seriously wounded in the shocking gun attack near Congress that killed a provincial official, Senate officials confirmed on Sunday night.
Acting Senate president Federico Pinedo confirmed Olivares, 61, had died of his wounds earlier in the day.
"The death of Deputy Olivares is a great pain," Pinedo tweeted.
President Mauricio Macri decreed a 48-hour period of national mourning.
At least six people have been detained in what has been described as a "mafia-style" attack Thursday that shocked the nation.
Officials have said two gunmen in a parked car shot wounded Olivares and killed Miguel Marcelo Yadón near the country's congressional building in Buenos Aires.
Yadon, 58, was hit five times at close range as he walked in a square in front of the National Congress with Olivares, a childhood friend.
The national deputy was struck in the abdominal area and his condition had been touch-and-go ever since.
Olivares, a member of the pro-government Radical Civic Union party, represented La Rioja province in the Chamber of Deputies, the lower house of congress. Yadón worked for La Rioja's federal electric transportation system.
Authorities have said investigators are working to determine a motive, but most signs seem to be pointing toward a personal dispute, with officials saying the motive was "not political" in nature.
Press reports in Argentina have claimed it may have been linked to a relationship that Yadon was having with a younger daughter of one of the assailants.