Lieutenant Patrick Hannon was rushed to Mercy Medical Center but he could not be revived.
Hannon, 51, joined the department in 1988. He was married and has two children. His wife is a firefighter paramedic with the department.
On Saturday afternoon, bunting hangs at the fire station on South Throop where Hannon worked.
Fire commissioner Robert Hoff offered his "support and prayers" to the Hannon family.
The cause of death has not been determined.