The blaze broke out on Ryton Street at Unique Fitness Gym at around 5am (Monday, February 13).
Nobody is thought to have been hurt as the gym is not currently open for business.
At the peak of the incident, there were six fire engines in attendance with Nottinghamshire and South Yorkshire crews working together.
Firefighters used hose reel jets and an aerial platform to extinguish the blaze and expect to be dampening the area down for the rest of the day.
Ryton Street has been re-opened but Newcastle Avenue remains closed.
Worksop station manager Richard Cooper said the cause of the fire is not yet known.
He added: “Even though Ryton Street has been re-opened, we would ask that motorists avoid this area.”
Keep checking our website for updates.