Treacy was already locked in until 2026, but his form has been so hot this year that the Dockers were keen to secure his services for much longer.
The new four-year extension means Fremantle's star-studded forward trio of Treacy (2030), Jye Amiss (2029) and Luke Jackson (2029) are all locked away for the long term.
Treacy, a rookie selection in 2021, was merely a solid contributor last year with 15.6 from 17 matches.
But a huge off-season has resulted in Treacy exploding to life this year, with his 42.15 placing him sixth in the Coleman medal race.
Treacy's contested marking and brutal tackling have also been a feature of his game, with his increased fitness levels allowing him to roam further up the ground and for longer.
The rise of Josh Treacy has been nothing short of amazing, and now the 21-year-old is in purple until 2030 💜— Fremantle Dockers (@freodockers) From country Vic to the big stage, his journey is only just beginning. #foreverfreoAugust 1, 2024
The 21-year-old, whose shot accuracy of 70 per cent this season is the best out of the top 40 goalkickers this year, was happy to secure his long-term future.
"It was clearly a no-brainer with the situation the club is in and the way that I feel it is going to go," Treacy said.
"We've got a really big month ahead of us and hopefully we can go further."
Fremantle (12-6-1) sit in third spot, but their final four games of the regular season will pit them against finals contenders.
The Dockers take on Essendon at the MCG on Sunday, before facing Geelong (home), GWS (Giants Stadium) and Port Adelaide (home).
Fremantle will be without the suspended Nat Fyfe against a Bombers outfit fighting for their finals survival.
Essendon (10-8-1) have lost six of their last eight games - including a disappointing 53-point loss to St Kilda last week - to slide to 10th.
The Bombers have been widely panned since losing to St Kilda in meek fashion, and Longmuir is wary of an emotional rebound.Â
"They've copped a fair bit in the media and the coach wasn't happy with them after the game," Longmuir said.
"I expect them to come out really physical and at their best.
"They've played some really good footy this year and we can't get caught up in last week's game.
"They've been in every game this year, apart from last week ... and they've challenged really hard."
Longmuir hinted he would send someone to tag Essendon star Zach Merrett, who is averaging 28 disposals and 4.7 clearances per game this season.
"He's had influence in games no matter what - wins and losses, tagged or not tagged," Longmuir said.
"He's a good player, so we can't just let him run around by himself."
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