The fire at Area on Burgess Street was reported at 1.20am and when firefighters arrived the former club was heavily smoke-logged.
South Yorkshire Fire and Rescue said firefighters had to search the club to establish the exact location and cause of the blaze.
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There are no reports of injuries and crews have urged people to stay away from the area.
A fire service spokesman said: “Crews were initially called to a disused nightclub on Burgess Street at 1.20am this morning.
“Since then they’ve been searching the club to find the fire and put it out – which is an on-going process.
“We’ve now got eight fire engines in attendance. This is mainly due to needing personnel for breathing apparatus duties and procedures.
“It’s still an on-going incident.”