The 11-time Grand Slam champion disclosed his diagnosis in his autobiography 'Heartbeats: A Memoir', co-written with wife Patricia, saying he underwent surgery in 2024 and is now in remission.
Borg won five successive Wimbledon titles between 1976 and 1980 as well as six French Open crowns, but stunned the sporting world by retiring at the age of 26.
"I spoke to the doctor and he said this is really, really bad," Borg told the BBC.
"He said you have these sleeping cancer cells, it's going to be a fight in the future. Every six months I go and test myself. I did my last test two weeks ago. It's a thing I have to live with."
Borg said doctors discovered the cancer in 2023 despite years of regular check-ups.
"The thing is that you don't feel anything - you feel good, and then it's just happened," said the Swede, who held the world number ranking for 109 weeks.
"I hope that I'm going to be OK. I take it day by day, year by year, hopefully."
Although Borg attempted a brief comeback in 1991, he failed to make a mark on the circuit.
During his prime, Borg shared an intense rivalry with John McEnroe and their battle in the 1980 Wimbledon final is considered one of the greatest ever tennis matches. The American won the fourth set tiebreak 18-16 before Borg went on to win the five-set epic.
"Now I have a new opponent in cancer - one I can't control," Borg wrote in his book.
"But I'm going to beat it. I'm not giving up. I fight like every day is a Wimbledon final. And those usually go pretty well, don't they?"