A spokesman for BMI The Huddersfield Hospital said: "We can confirm that the hospital is temporarily closed while the police investigate an incident which took place outside the hospital.
"We hope to reopen again tomorrow and have been in touch with patients who were due to attend."
Callers to the hospital's reception are being directed to a recorded message which says: "Due to a major incident, the BMI Huddersfield Hospital will be closed until seven o'clock Thursday the 23rd of August.
"If you have an urgent clinical query, please phone this number to speak to our clinical on-call team - 07824 450975."
More to follow.