NHS Grampian is reducing the number of inpatient mental health wards at Royal Cornhill Hospital in Aberdeen due to staff shortages.
The health authority, which has more than 50 mental health nursing vacancies, said the change would take effect from Thursday.
NHS Grampian admitted the situation would be “unsettling”, but that it expected staffing levels to improve.
The change will see the number of wards reduced from six to five.
Jane Fletcher, head of hosted mental health and learning disability services, said: “Any change is unsettling but the safety of patients and staff is our main priority.
“This is a temporary move and we expect staffing levels to improve in the coming months as graduate nurses take up employment at the hospital.”
However, Simon Watson, the Unison union’s regional organiser, said: “Short staffing is nothing new.
“We are sickened that staff shortages have resulted in this closure and vulnerable adults in Grampian may suffer as a result.”