The former England international has been absent recently due to a calf complaint and he broke down in training on Saturday when he twisted his right knee while turning.
Dr Peter Florida, QPR's head of medical services, told the club's website: "JJ has completely ruptured the anterior cruciate ligament in his right knee.
"We'll be taking him to see a specialist later this week to decide the next course of action.
"At this stage it would appear he'll require a full repair, which is likely to keep him out for between 6-9 months.
"JJ is in fairly good spirits, all things considering.
"He's been unlucky throughout his career with injuries, but he's in a positive frame of mind.
"JJ does everything asked of him in terms of injury prevention techniques but, unfortunately, he's just one of these very unlucky players when it comes to injuries."
The 31-year-old Jenas, who has previously had spells with Nottingham Forest, Newcastle and Tottenham, joined QPR on the final day of the January 2013 transfer window.
Capped 21 times by England earlier in his career, he has made 36 appearances for QPR, scoring four goals.