Nuno Tavares 22-year-old left-back is set to have a medical with the French side on Friday with an official announcement pending.
The Portuguese youth international, who started 13 Premier League games for the Gunners last season, saw a proposed move to Atalanta fall through at the start of the week.
The loan deal will not include an option for Igor Tudor's side to purchase Tavares, and the French club will pay his wages throughout the loan spell.
Tavares arrived at Arsenal last summer in an £8million deal after impressing in the Primeira Liga at Benfica.
After playing in every game of the the club's run to the EFL Cup semi-finals last season, Tavares was left on the bench as Mikel Arteta's side crashed out to eventual winners Liverpool.
With Kieran Tierney serving as Mikel Arteta's preferred option on the left-side of defence, and the arrival of Oleksandr Zinchenko earlier this month, opportunities at the Emirates looked to be limited for Tavares ahead of the upcoming campaign.
Marseille are also in talks to sign former Arsenal and Manchester United forward Alexis Sanchez from Inter Milan.
Sanchez has found himself shuffling down the pecking order at Inter since joining the club in 2020 following his horror spell in Manchester, but a move to the Stade Velodrome would offer the former Arsenal star Champions League football and the prospect of more minutes.
The Chilean international is said to be interested in a move to Marseille but the deal is said to be hinged on 33-year-old coming to an agreement with Inter on severance pay.