The 20-year old defender who was brought from St Etienne in 2019 for a £27million fee is yet to play his first game for the Gunners.
He spent the second half of last season on loan with Nice and would be back to his home country to play another season with Marseille.
Edu told the club’s website: “Together with William, we have decided it will be good for his continued development to spend another season on loan.
“William joined us as an 18-year-old, and he is still only 20, so he is still developing all the time.
“William is a player with strong natural ability. Next season has the potential to be really beneficial for him at Marseille, a good club.
“To play another season in Ligue 1 will be very important for his development.”
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