Kelly was released from the hospital last week after undergoing treatment for sinus cancer.
In a baseball cap and sunglasses, the Hall of Fame quarterback told the current Bills squad that it was part of a "new era" and it was time to "start playing games in January."
Bills wide receiver Mike Williams said Kelly's visit was emotional.
"To be honest, I'm not no little girl, but that brought a tear to my eye. That's a legend right there," he said. "It was a joy to see him out there, talking and walking around and to come and talk to us and to say the things he said about our offense, it was kind of like a joy."
The Bills took to Twitter to show Kelly addressing the team.
Jim Kelly addressing the team pic.twitter.com/IzNBbSPbfO
- Buffalo Bills (@buffalobills) June 19, 2014
Kelly led the team in a group huddle before returning to a chair on the sideline to watch practice.
As Kelly continues to recover from chemotherapy and radiation, doctors are expected to wait several weeks before determining if he needs surgery.
Kelly was unable to attend his annual charity golf tournament earlier this month but his wife, Jill, tweeted that he plans on attending his football camp next month.