Around 40 firefighters battled the blaze at The Happy Wanderer, in Bilston, on Thursday night.
Greens Lane was closed to traffic while firefighters tackled the flames.
They operated hoses from a hydraulic platforms, dosing the charred building with water. Parts of the roof were left gutted.
A spokesman for West Midlands Fire Service said: "We had a call at 7.58pm to The Happy Wanderer.
"The first crew got there in three minutes. We had six crews in attendance, along with the hydraulic platform, which is around 35 to 40 personnel.
"We do believe it was started deliberately. The building was derelict, which crews said was well alight when they arrived.
"Crews had to split the firefighting operation into sections. The gas and electricity board was in attendance. Greens Lane was closed to the public.
"Our firefighters carried out a thorough search of the building using breathing apparatus and thermal imagining cameras.
"Police were making investigations as to who the owner of the building was."
West Midlands Police have been approached for comment.
It was the latest pub fire to happen in the Black Country.
Earlier this month, a fire broke out at the former Fiddlers Arms, in Gornalwood, Dudley, which was believed to have started deliberately.
Last month, the derelict Travellers Rest, in Great Bridge Road, on the border of Bilston and Moxley, was set alight in a suspected arson attack.