The fire was lit within a secure ward at the hospital about 10am today, a Fire and Emergency New Zealand (FENZ) spokesman said.
Appliances from Willowbank, Roslyn and St Kilda attended the blaze.
"The fire is suspicious and police are at the scene doing an investigation," the Fenz spokesman said.
A Southern District Health Board spokeswoman said eight staff were assessed for smoke inhalation - three were taken to Dunedin Hospital's emergency department, and a fourth went there later in the day.
One patient was checked at the scene, and determined to not have any injuries.
She said the fire was discovered by staff, who immediately extinguished the blaze.
St John said they were told by FENZ a couch at the facility was set on fire. A Fenz spokesman said he knew the cause of the blaze and what was set alight, but could not comment.
A fire investigator was not required.
A police spokeswoman confirmed the fire was within one of the hospital's wards.
"Inquiries are ongoing to determine how it began."
When asked if the fire was suspicious, she said police "can't confirm that at this stage".
The SDHB spokeswoman said Wards 9A and 9B were back to business as usual ''a very short time'' after the incident.
The cause of the fire was being investigated.
