Two men were shot dead in a bar in the centre of the port city late on Monday in an apparent settling of scores, police said.
One of the dead men, 41-year-old Patrick Esposito, was known to police, but the other victim was a barman who had no criminal links, Marseille prosecutor Xavier Tarabeux said.
Murders linked to illicit drug dealing in the Marseille area totalled 19 last year but this summer has been especially deadly, with 10 people killed since August 7th.
Police admit they are almost powerless to prevent the drug murders.
"These settling of scores are very hard to prevent," an official of the police union Alliance said last week. "They can happen anywhere, any time."
France's second largest city, the Mediterranean port has a long-running reputation for violence linked to the drugs trade.
Authorities attribute this year's spike in drug-related violence partly to a surge in police operations to dismantle competing drug-trafficking rings.
Interior Minister Bernard Cazeneuve has said that police seize some 6,000 weapons a year from criminal groups in Marseille, including 1,200 combat rifles and other military weapons.