Fire crews and police were called to the incident at around 5.10am.
The accident and emergency department at the hospital is currently closed.
A Nottinghamshire Fire and Rescue spokesman said: “We are currently in attendance at a suspected chemical incident at Bassetlaw Hospital.
"We have now begun the process of scaling back the incident but a presence maybe there for some time.
“The emergency department is currently closed at the hospital however the far side remains open.”
Doncaster and Bassetlaw Teaching Hospitals NHS Foundation Trust has also released a statement about the incident.
It said: “Colleagues are currently dealing with an incident within the emergency department at Bassetlaw Hospital, which means the service is temporarily closed.
“The area directly in front of the main entrance is also not available for parking - alternative arrangements have been made within the triangle car park.
"We are currently working closely with partners and will update shortly when this matter has been resolved.