Salmon made 11 appearances for Celtic this season, scoring three goals, including his first league goal for the club in their 3-1 win over Morpeth last month.
He had originally signed on loan until the end of January, 2021.
The club added: "Celtic would like to thank Lewis for his hard work this season and we wish him all the best in his future career."
Keep up-to-date with our exclusive email newsletters.
Digital advertising by Minute Media
© Pitch Hero Ltd 2008-2020
Pitch Hero Ltd Registered in ENGLAND | WF3 1DR | Company Number - 636 1033