Two flashing headers from Ludovic Ajorque in the first-half fired Brest to a 2-0 home win as Marseille got off to a losing start under new coach Habib Beye in Ligue 1 on Friday.
Former Marseille captain Beye only took over from Roberto de Zerbi on Wednesday in one of France's most passionately supported clubs.
After a journey of some 1276 kilometres (800 miles) from the Mediterranean coast to the western tip of Brittany on the Atlantic, Marseille dominated passing and possession. But this fourth game without a win hurts their push for Europe.
Brest's 6ft 5in (1.96m) centre-forward Ajorque opened the scores with a towering header on 10 minutes and repeated the feat just before the half-hour.
Mason Greenwood was Marseille's stand-out performer, but when he won a penalty in the 81st minute the Brest 'keeper Gregoire Coudert dove the right way and saved.
Marseille remain in fourth place in Ligue 1 on 40 points, five short of Lyon who sit in third, the last of the three direct Champions League spots.
Lyon head to Strasbourg on Sunday on a run of 13 consecutive wins, seven of them in Ligue 1.
Leaders Lens have won their last 10 home games in Ligue 1 and a victory against Monaco on Saturday will set a new club record of 11.
Paris Saint-Germain, who are one point behind Lens in second, host rock-bottom Metz in Saturday's late game.