The legendary Green Bay Packers quarterback made the announcement during a welfare reform hearing in D.C. Tuesday ... where Favre said he believed he was on the verge, with the company, Prevacus, of developing medication to help treat head injuries.

"Sadly, I also lost an investment in a company that I believed was developing a breakthrough concussion drug I thought would help others," Favre said during his opening statement.

That's when Brett revealed he'd be diagnosed with the degenerative nervous system disorder.

"And I'm sure you'll understand why it's too late for me, because I've recently been diagnosed with Parkinson's. This is also a cause dear to my heart."

According to the National Institute of Aging ... Parkinson's "is a brain disorder that causes unintended or uncontrollable movements, such as shaking, stiffness, and difficulty with balance and coordination."

During an interview in 2018 ... Favre said he only knows of "three or four" concussions he has suffered ... but that he could have suffered more than 1,000 over the course of his 20-year career.