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.
A hot tub is thought to have been the probable source of an outbreak of Legionnaires' Disease, the Health Protection Agency has said.
Based at JTF Warehouse in City Road, Fenton, Stoke-on-Trent, samples taken from the hot tub were found to have an unusual strain of legionella bacteria, which matched that of the strain taken from patients with the illness.
Dr Sue Ibbotson, regional director of Health Protection Agency, West Midlands, said the organisation has taken histories from those with Legionnaires and 17 of the 18 confirmed cases visited the warehouse in the two weeks before they fell ill. One patient died after being affected by the disease.
Public and environmental health experts from the Health Protection Agency, the Health and Safety Executive, the NHS in Stoke and Stoke-on-Trent City Council are jointly investigating the seven confirmed cases.
There are seven confirmed cases of Legionnaire's disease in Stoke-on-Trent.
The seven patients are being treated at University Hospital of North Staffordshire.
One man has died following an outbreak of Legionnaires' disease in Edinburgh. Health officials say a further 15 people are in a critical condition and 15 suspected cases are being investigated.
The patient who had underlying health conditions was being treated at the Royal Infirmary of Edinburgh.