No casualties were reported but pictures showed severe damage to the 150-year-old building.
Several patients were placed in an ambulance area on mattresses, being tended by nurses who covered them with extra blankets. Other patients wrapped in blankets were led in wheelchairs to safety. Officials said they did not immediately know how many patients were evacuated.
Seventy-five firefighters fought the blaze as smoke billowed from the scene.
The Royal Marsden was the world's first hospital to be dedicated to the study and treatment of cancer, welcoming patients from around the world.
"It's unbelievable what would happen if it burned down. It would be a huge, huge step back for all the patients being treated there, and a huge, huge step back for cancer," said Professor Ray Powles, former head of hemato-oncology at the hospital.
