The 82-year-old Specter issued a brief statement through his Philadelphia office.
Action News has learned that Specter is currently at the Hospital of the University of Pennsylvania.
Specter had "a big flareup" of the disease Monday night, a source told CNN.
Specter, who with 5 terms was Pennsylvania's longest-serving senator, has overcome two bouts with Hodgkin's disease, a cancer of the lymphatic system, since 2005. He also has survived a brain tumor and cardiac arrest after bypass surgery.
Sources tell Action News that the latest form of cancer to affect Specter was diagnosed about 6 weeks ago. His family is closely shielding specifics about his condition.
In his statement, he said: "I'm battling cancer. It's another battle I intend to win. I'm grateful for all the well wishes I've received. I'm looking forward to getting back to work, to the comedy stage, to the squash court and to the ballpark."
In retirement, among other activities, Specter occasionally performed standup comedy at clubs in Philadelphia and New York.
He played squash nearly every day into his mid-70s.
Specter has also been practicing law, writing books and teaching.
His 30-year career ended after he switched from the Republican Party to the Democratic Party. He lost the Democratic primary in 2010, and Pennsylvania voters elected conservative Republican Pat Toomey from the Lehigh Valley, who was a U.S. Congressman at the time, to replace him.
In Tampa Bay, Florida, at the Republican National Convention, Specter's former colleague Rick Santorum, who was a member of the U.S. Senate with Specter, said he learned about Specter's illness about a month ago.