The Wyoming Republican, who was easily elected to a third term while in the hospital, issued a statement saying he will undergo treatment and plans to return to Congress in January.
Thomas fell ill in church in Casper on Sunday and was taken by ambulance to Wyoming Medical Center. He and his wife, Susan, flew back to Washington later in the day.
Thomas, 73, canceled campaign events Monday and was admitted to the National Naval Medical Center in Bethesda, Md., that evening, his spokesman Cameron Hardy said Thursday.
While hospitalized, doctors noticed a low white blood cell count and a bone marrow biopsy revealed leukemia, cancer of the blood and bone marrow. Thomas was to immediately begin a four- to six-week course of chemotherapy and will remain at the hospital to reduce his risk of infection, his office said in a press release Thursday.
"I am joining all of the thousands and thousands of other cancer survivors in Wyoming," Thomas said. "I certainly didn't expect this diagnosis, but I will be back by the January session ready for full service in the new Congress."
Thomas was diagnosed with acute myeloid leukemia, the most common form of the disease.
Dr. Brian Monahan, director of hematology and medical oncology at the National Naval Medical Center, said in the release that the disease usually can be forced into remission in people in Thomas' "great physical condition."
Thomas was elected to the Senate in 1994 after serving five years in the U.S. House.