"NOTHING MORE THAN A MIRACLE!!! THANK YOU GOD!!!," he wrote Thursday on wife Jill's Instagram account.
Kelly, 54, had just learned that follow-up tests revealed no signs of the sinus cancer he has tackled since March.
"I don't even know what to do with myself," Kelly said. "I'm so overwhelmed.
"I'm so thankful. I want to literally hug and thank all of you in person."
Among the first things did was phone his daughter, Erin Marie, to tell her the good news:
Kelly received the news from Dr. Peter Costantino, a week after he had a series of biopsies performed at Lenox Hill Hospital in New York.
Costantino, a head and neck specialist who has overseen Kelly's cancer treatments since March, told him the results confirmed what he believed to be the case following an initial physical examination two weeks ago.
"Dr. Costantino said it was nothing short of a miracle because he wasn't sure when he went in what was going to come out," said Kelly, who completed chemotherapy and radiation treatments three months ago.
"This is some of the greatest news I've ever gotten."
Kelly's very public battle with cancer began in May 2013, when he had cancerous cells (squamous cell carcinoma) surgically removed.
The procedure involved the removal of a portion of his upper jaw and several teeth, replacing them with a prosthetic jaw and false teeth.
But Kelly later began suffering headaches and learned in March that the cancer had returned and spread aggressively to his sinuses. Further surgery was considered too risky because of where the tumours were located.
Treatments sapped his boundless energy somewhat, but he remained strong enough to attend last month's Hall of Fame ceremony in Canton, Ohio, where fellow Buffalo Bills stalwart Andre Reed closed his induction speech by asking Kelly to throw him one final pass.
It was a touching moment and evoked memories of the uncanny chemistry the Hall of Fame duo displayed on the field, connecting on 663 completions with Buffalo before Kelly retired on Feb. 1, 1997:
The next day after being inducted, Kelly served as Buffalo's honourary captain in the Hall of Fame game, losing 17-13 to the N.Y. Giants.
On Thursday, Jim Kelly came up a winner.