The 51-year-old Spaniard said the tumor was detected after "exhaustive tests." A biopsy is scheduled for Tuesday.
Ballesteros collapsed at a Madrid airport last week and was hospitalized.
"Throughout my whole career, I have been one of the best at facing obstacles on the golf course," Ballesteros said in a statement on his Web site. "Now I want to be the best, facing the most difficult match of my life, with all my strength, and counting on all of you who are sending encouraging messages."
Ballesteros won the British Open three times and the Masters twice.
The Spaniard won 50 times on the European tour and has 87 titles worldwide. He appeared in eight Ryder Cups, accumulating a record of 20-12-5.