Schilling announced the news on social media and thanked his Boston-based medical team.
He revealed his diagnosis in February, saying he planned to ''embrace this fight, just like the rest of them, with resolute faith, and head on.''
He hasn't indicated the type of cancer or his prognosis, but has posted pictures of himself undergoing treatments.
Schilling is a three-time World Series champion - with the Arizona Diamondbacks in 2001 and the Red Sox in 2004 and 2007. In 20 seasons, he was 216-146 with a 3.46 ERA. The right-hander shared the 2001 World Series MVP award with left-hander and teammate Randy Johnson.
After pitching in the 2007 World Series, Schilling suffered reversals on and off the field. His 2-1 victory in Game 2 of Boston's four-game sweep of the Colorado Rockies was his last game.
He spent the entire 2008 season on the disabled list after right shoulder surgery and retired in March 2009. He started an ill-fated video game company, 38 Studios, and later said he invested and lost as much as $50 million. Its collapse is the subject of a lawsuit in Rhode Island after it got a $75 million state-guaranteed loan.
On Feb. 5, Schilling announced in a statement issued by ESPN that he was diagnosed with cancer.
''With my incredibly talented medical team I'm ready to try and win another big game,'' the statement said.
On May 28, Schilling received a loud ovation from Fenway Park fans during a pregame ceremony honoring members of the 2004 team that won the Red Sox first championship in 86 years. Like the other players, he walked in from the left-field wall. His son Gehrig was by his side.