Treacy has managed eight goals so far this season, including two goals in a career-best game against Sydney, where he also had 14 disposals, six marks and four tackles.
Head of Player Personnel at Fremantle David Walls said it was an easy decision to re-sign Treacy.
"Josh is loving life at Fremantle, and we were happy to see him commit for the three years," Walls said.
"Josh can see a long-term future for himself at Fremantle and he was keen to commit and just wanted to repay the Club."
"He's loving his time in West Australia, loving his time with his teammates and coaches and it was relatively straightforward to get the contract done."
Treacy has overcome an ankle injury, glandular fever and form issues to earn his new contract, with a stint for Fremantle's WAFL side Peel from Rounds 2-7 reigniting his love for the game.
"I'm extremely grateful and excited at the same time for the opportunity and the faith the Club have shown to me," Treacy said.
"I've had a bit of an up and down three years, but I'm certainly on the way up and hopefully I can continue to build for another three years."
"It's been a bit of a challenge but I think for the better. Without having gone through those issues, I don't know if I'd be in the situation I am right now."