Fermor is coming off the back of a breakout season, crossing for 11 tries in 23 appearances, as well as being included in Queensland's extended squad, and debuting for the Prime Ministers XIII against Papua New Guinea post-season.
Forming a dynamic back-row combination with David Fifita, Fermor's injury leaves a gaping hole in the Gold Coast's forward pack just 72 hours before their first match of the season.
The club confirmed the news on Thursday afternoon.
"The Gold Coast Titans can confirm Beau Fermor has been dealt a devastating injury blow, ruling the talented edge forward out of Sunday's season opener against the Tigers," the club said in a statement.
"Scans have revealed the 24-year-old has ruptured his ACL at training this week with the back-rower to now meet with medical staff to outline his rehabilitation.
"Facing a lengthy stint on the sidelines, the Titans wish Fermor the best in his recovery."
It isn't the first ACL injury suffered by Fermor, who tore his anterior cruciate ligament whilst plying his trade for Newcastle's Jersey Flegg team in 2019.