Ward is shut after vomiting outbreak.
Powys Teaching Local Health Board has shut the Graham Davies ward at Llanidloes hospital until further notice.
Several wards at Withybush hospital in Haverfordwest have been closed following a similar outbreak last week.
Staff at Llanidloes hospital said they were taking all the necessary precautions to ensure the care and safety of patients.
Powys Teaching Local Health Board said people with symptoms of diarrhoea or vomiting should stay at home.
A health board spokesman said: "It is particularly important that people who are unwell keep away from patients in hospitals and long-term care facilities as they may be at increased risk of contracting gastroenteritis.
If they do contract it they may experience more severe symptoms and it may exacerbate their other health problems."