Test on Monday confirmed that the the 22-year-old, who was hurt in Argyle's 1-0 loss at Barnet last week, will be out for at least six months.
The ex-Bristol Rovers player Tweeted that his injury was 'not looking too promising' last week.
Brunt injured the cruciate ligament in his other knee in 2011 and had further knee surgery in 2014.
Brunt, whose contract expires in the summer, had just established himself in Argyle's first team, starting in each of the five games he played in before his injury, scoring three times in the process.
Plymouth Argyle striker Ryan Brunt could be out for nine months after injuring his cruciate knee ligament.