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The 26-year-old Milik has not played for Napoli this season and joins a Marseille side who have slipped off the title pace in France in recent weeks.
Media reports in France suggested Marseille could opt to make the deal permanent for eight million euros ($9.7 million), with four million euros in possible bonuses.
Milik, who joined Napoli from Ajax in 2016, scored 48 goals in all competitions for the Serie A side.
A three-match winless run has left goal-shy Marseille sixth in Ligue 1, 10 points behind leaders Paris Saint-Germain and second-placed Lille.
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Coach Andre Villas-Boas has come under increasing pressure and said he would leave the club if asked to by the club board following Wednesday's 1-0 home loss to Lens.
The Portuguese will be hoping that Milik can help his team rediscover their best form, with Florian Thauvin, who has been heavily linked with a move away from the Stade Velodrome, their top-scorer in the league this season with only six goals.
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