Patients, staff and visitors were evacuated from Wishaw General Hospital in North Lanarkshire earlier.
Pictures on social media showed elderly patients on trollies outside the main building.
Police were called to the site at about 16:40 after an alarm was activated. The building was assessed as secure and the alert is believed to have been caused by a fault with the system.
NHS Lanarkshire said all patients were now back inside.
A spokeswoman for Police Scotland said: “Police officers attended and a search was carried out in conjunction with hospital security staff. The building was assessed as secure it was believed to be a system fault with the alarm.
“There was no impact on the health and safety of patients and normal hospital service resumed.”
Calum Campbell, chief executive of NHS Lanarkshire, said: “At around 4.40pm, our alarm system at Wishaw General Hospital was triggered. Staff followed protocol, emergency services quickly attended and there was a partial evacuation of the building.
“Within the hour we had received confirmation that it was safe to re-enter the building and the hospital quickly returned to normal.
“We are investigating the cause of the alarm which is believed to be due to a fault with the system. We are sorry for the inconvenience caused to patients and visitors and are pleased to report that there was no impact on the safety of patients as a result of this incident.”