The killings are drawing attention to the ruling Socialists traditionally weak issue of security as President Francois Hollande's government struggles to control crime in France's toughest city.
Adrien Anigo was the second person killed in the region of the southern port city on Thursday, bringing the number of murders to 15 since the start of the year.
Anigo, about 30, suffered multiple gun shots, including one to the head, as he was targeted getting out of his car by two gunmen on motorcycles who fled the scene.
His killing came hours after gunmen shot a 24-year-old as he arrived for work in the town of La Ciotat, some 30 kilometres (20 miles) from Marseille.
Prime Minister Jean-Marc Ayrault ordered 130 extra riot police and 24 investigators last month to help stem killings in Marseille, which has France's second-biggest population and a large immigrant community.
Most victims were involved in drug trafficking and shot by rival gang members.