"It's terrible luck for Fish and a real setback for us," said Warriors coach Matt Elliott. "He had been doing a terrific job for us.
"Naturally he's devastated and we all feel for him. What we'll do as a group is give him all the support we can to ensure he recovers well for next season."
Fisiiahi, who has had his best season since his NRL debut in 2011, left the field in the 60th minute of the club's 24-14 win over the Tigers.
After making eight appearances in 2011 and 2012, he more than doubled his career tally to 19 matches, scoring five tries in 11 games this year.
Expected to take Fisiiahi's spot on the right wing is 20-year-old Ngani Laumape, who has made eight NRL appearances since his debut against Cronulla in March.
Fisiiahi becomes the fourth Warrior with a season-ending injury. Sione Lousi was ruled out for the year with an ACL knee injury in the first trial of the season in February, while Steve Rapira and Ben Henry also needed surgery to repair ACL injuries later in the year.
Apart from that quartet, the Warriors have Russell Packer (hamstring) and Charlie Gubb (groin) on the injured list, though it's hoped they will be available for selection when the Warriors host the Storm on Sunday.
#
Walking wounded - Warriors' injury list:
Glen Fisiiahi - pectoral (out for season)
Charlie Gubb - groin (back for round 20)
Ben Henry - knee (out for season)
Sione Lousi - knee (out for season)
Russell Packer - hamstring (back for round 20)
Steve Rapira - knee (out for season)