The statement, shared by Newcastle, read: "Kevin Keegan was recently admitted to hospital for further evaluation of ongoing abdominal symptoms.
"These investigations have revealed a diagnosis of cancer, for which Kevin will undergo treatment.
"Kevin is grateful to the medical team for their intervention and ongoing care.
"During this difficult time, the family are requesting privacy and will be making no further comment."
Newcastle wished Keegan, 74, "a full and speedy recovery" on X, adding: "King Kev. We're with you every step of the way."
Keegan played for Liverpool, Hamburg, Southampton and Newcastle, among other clubs, during his iconic playing career.
The striker also captained England and scored 21 goals in 63 games for the Three Lions.
He managed Newcastle twice, going close to winning the Premier League in the 1995/96 season.
He became England manager in 1999 but left the role the following year.