Emergency services attended a structure on Lodge Road shortly after 17:15 BST on Sunday evening.
The building, formerly known as Coleraine High School, had been closed and stood empty at the time of the incident.
Firefighters from the Northern Ireland Fire and Rescue Service (NIFRS) deployed five appliances to tackle the blaze.
It took just under four hours for crews to extinguish the flames and secure the site.
The road was temporarily blocked to facilitate access for the firefighting operations.
Officers from the Police Service of Northern Ireland (PSNI) arrived at the scene alongside the fire service personnel.
A statement released by the PSNI indicated that no injuries were reported among anyone on the ground.
Authorities have now classified the event as an act of arson, ruling out accidental causes.
The former all-girls' grammar school building remains the focus of this investigation into the deliberate ignition.
No further details regarding the cause or potential suspects have been provided at this stage.
The site is currently under police control while a full inquiry proceeds.