The Health Protection Agency said the latest case brings the total to 21. The patient, who is being treated at the University Hospital of North Staffordshire, is a man in his 70s.
Two patients being treated for the disease since the spate of illnesses began in the middle of July have died.
People who contracted Legionnaires' disease this year are calling for a public inquiry after a fatal outbreak in Stoke-on-Trent.
A second person has died after an outbreak of Legionnaires disease in Stoke-on-Trent.
Officials believe the Legionnaires outbreak at Stoke On Trent can be traced to a hot tub in a Stoke warehouse.