Forrest arrived at Parkhead when he was just nine-years-old and came through the Celtic Youth Academy. He has since made 193 appearances, scoring 31 goals.
The winger had previously been linked with a move away but scored his side’s opening goal of the season in the 2-1 victory over Hearts on Sunday.
He faces increased competiton after the addition of Scott Sinclair, who also netted at the weekend, though has been given the chance to impress his new manager after Patrick Roberts limped off during the match versus Astana with a hamstring injury.
The news comes the day after Celtic announced that Tom Rogic had signed a new deal, also keeping him at the club until 2019.