The deal is reportedly worth up to £17 million, comprising an initial £15.3m fee and a further £1.7m in performance-related add-ons.
With only one year remaining on his contract, Arsenal have chosen to cash in, recouping much of the £27m they paid Brighton to sign him in January 2023.
Trossard leaves the Premier League champions having scored 36 goals in 174 appearances across all competitions, establishing himself as one of Mikel Arteta's most dependable attacking options.
According to Sky Sports, 17 of his 27 Premier League goals either put Arsenal ahead or drew them level in matches, with only Bukayo Saka scoring more goals of that significance for the club since Trossard's arrival.
His defining contribution came in May, when his dramatic late winner against West Ham proved a pivotal moment in Arsenal's march to the Premier League title.
He ended the campaign with eight goals and 11 assists in 50 appearances as Arsenal secured the Premier League crown for the first time in 22 years and reached the UEFA Champions League final, where he set up Kai Havertz's opener before the Gunners were beaten on penalties by Paris Saint-Germain in Budapest.
Trossard's departure is expected to form part of a wider reshaping of Arsenal's options on the left wing.
Aston Villa's Morgan Rogers reportedly remains the club's priority target, while PSG's Bradley Barcola is also admired.
Club Brugge's Christos Tzolis has been monitored as another potential option, although the Greek international is expected to command a fee in the region of £34m, which would represent a Belgian Pro League record.
Trossard also leaves having contributed 34 assists alongside his 36 goals, underlining the consistency of his output since arriving from Brighton in January 2023.
On the international stage, Trossard enjoyed another productive summer, starting all six of Belgium's matches as they reached the FIFA World Cup quarter-finals, scoring twice during the tournament.
According to the BBC, Arsenal are expected to step up their pursuit of Club Brugge forward Tzolis following Trossard's departure.
The Greece international has emerged as a leading candidate to strengthen the left side of Arteta's attack, with the Belgian champions anticipating an approach.
The Gunners also retain a long-standing interest in Atletico Madrid forward Julian Álvarez, though any deal is expected to be difficult given competition from Barcelona and Real Madrid.
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