Huddersfield BMI Hospital will be closed until tomorrow morning, with the area outside the hospital also cordoned off by police.
Patients have received calls from the privately-run hospital telling them not to come in for appointments.
The exact nature of the incident has not been revealed at this stage.
A statement issued by the hospital said: “Due to a major incident, the Huddersfield BMI Hospital will be closed until 7am on Thursday August 23.
“If you have an urgent clinical query, please call our switchboard on 07824450975.”
West Yorkshire Police have said they are aware of an incident and will be releasing more information shortly.