The 31-year-old -- who held out from the team earlier this season -- returned to the Redskins this week ... and in his first meeting with reporters, he made the stunning revelation.

Williams claims back in 2013, he approached team doctors about a growth on his head ... but says after they told him it was a benign cyst and nothing to worry about, he essentially ignored it.

But, Williams says the tumor kept on growing ... and when he finally went to have it checked out by independent doctors this year -- they told him it was a serious form of cancer.

"[They] told me it was DFSP," Williams said, referring to Dermatofibrosarcoma Protuberans. "That's what the cancer is called. It's a very rare soft tissue cancer."

Williams said the form of cancer was so serious, "I almost lost my life" ... and added, "I was lucky."

The 6-foot-5, 320-pound offensive lineman said he needed multiple surgeries -- including one where surgeons extracted the tumor from his skull -- to fix the issue.

Williams says other than some lingering effects on his scalp due to the surgeries, he's relatively fine now ... but he says he's having some serious difficulties trusting the Redskins.

Williams says the alleged misdiagnosis from team doctors is the reason he's been away from the 'Skins this season ... and he didn't seem sure if he'd ever play for the team again.

"Seriously, I almost lost my life," Williams said. "You're 30, coming off your seventh straight Pro Bowl and a doctor tells you to get your affairs in order."

"It's not going to sit well with you. And, it still doesn't."