Thirty firefighters descended upon the old Heath Town swimming baths following reports of a fire at 6.20pm on Thursday.
Crews arrived at the Grade II Listed building within three minutes and found a fully-developed fire on the ground floor.
The fire had started to spread to the first floor, affecting around a third of the building in all.
Crews managed to bring the fire under control by around 7pm, and it was confirmed out by 9.30pm.
A spokesperson for West Midlands Fire Service said: "A severe fire at Heath Town’s disused swimming baths is believed to have been started deliberately.
"Thirty firefighters from six Black Country fire stations responded to the incident, which was reported to us at 6.20pm on Thursday.
"Our first crews arrived within three minutes of being mobilised to Tudor Road.
"They were met with a fully-developed fire on the ground floor which had started to spread to the first floor, affecting around a third of the building in all.
"In spite of challenges presented by the condition of the building, crews had made good progress in bringing the fire under control by around 7pm.
"It was confirmed to be out by 8.30pm, when firefighters were damping down remaining hotspots.
"Most of our resources had left the scene by 9.20pm.
"We made further visits throughout the night to ensure that the fire had not reignited."
A driver who was passing along Bushbury Road at the time reported seeing multiple fire engines and lots of black smoke.
Heath Town Swimming Baths and Library has been closed and the building unused since 2003.
Last year, plans to turn the dilapidated former swimming pool into a major conference, wedding and banqueting were unveiled.
Sitting empty since 2003, the council-owned property has become a regular target for vandals and potential looters.