Essex firefighters were called to the scene near the terminal shortly after 10.30am.
Staff say flights are operating as normal.
UPDATE 1035 UTC Fire now under control, no aircraft in the hanger.
A Southend Airport spokeswoman said: “Essex Fire and Rescue is currently dealing with an incident at the Air Livery hanger on the north side of the airfield.
“London Southend Airport is still open and operating as normal”.