On Friday afternoon, the Gunners unveiled their new winger after sealing a quickfire deal with Brighton, having previously missed out in their bid to sign Mykhailo Mudryk.
Nicolas Pepe was the last player to wear the No19 shirt for Arsenal before leaving on loan for Nice last summer.
Another winger, Santi Cazorla, held the jersey for a number of years after Jack Wilshere, Gilberto Silva and Junichi Inamoto - associating it with a number of fan favourites.
Arsenal paid an initial £21m with £5m agreed in add-ons to sign Trossard, 28, and hope he will be eligible to face Manchester United on Sunday.
A club statement read: “All documents have been submitted and we’re hoping Leandro will be eligible for selection for our match against Manchester United on Sunday.”
Mikel Arteta hailed Trossard’s arrival as a vital addition to his squad ahead of a big push to win Arsenal’s first title in almost 20 years.
He said: “It is good work from everyone at the club to complete the signing of Leandro. He is a versatile player with high technical ability, intelligence and a great deal of experience in the Premier League and at international level.
"Leandro strengthens our squad as we head into the second part of the season and we’re all looking forward to working with him. We welcome Leandro and his family to Arsenal Football Club.”