The actor, who is best known for playing Screech in the Nineties high-school sitcom, was hospitalised in Florida last weekend after feeling pain throughout his body.
His manager has now confirmed that Diamond, 44, has been diagnosed with cancer and is awaiting test results to learn the nature and location of the illness.
The representative also confirmed that Diamond had been experiencing pain from shingles.
A close friend of the actor told The Sun that Diamond has had a “huge lump” on his neck for some time but put off going to the hospital in case people took photos and posted them online.
Stage four cancer means that the illness has spread to other organs or parts of the body, with Diamond’s friend Dan Block stating that the actor is starting chemotherapy this week.
California-born Diamond began acting as a child. As the loveable nerd Screech, he was one of only two characters to appear as a Saved by the Bell series regular from the beginning of the franchise to the end.
He appeared in Good Morning, Miss Bliss, Saved by the Bell, Saved by the Bell: The College Years and Saved by the Bell: The New Class.
Screech went on to become the principal’s assistant at Bayside High and is mentioned in the recent reboot as having become an astronaut at the International Space Station.
However, Diamond did not return to the 2020 revival on streaming service Peacock along with his former castmates.