Fire crews spent more than four hours tackling a fire.
Crews at East Sussex Fire and Rescue were alerted to the incident at Sussex House, in Crowhurst Road, Hollingbury, before 6.30pm on Wednesday.
Six fire engines were deployed, including an aerial unit, and there were no reports of injuries.
At 9pm, the response was scaled back to two fire engines, before the scene was cleared at 11.38pm.
The cause of the fire at the two-storey building is thought to be accidental.
An East Sussex Fire and Rescue Service spokesman said: "East Sussex Fire and Rescue Service left the scene at Crowhurst Road after a fire inside the property.
"Crews used in total; four breathing apparatus, two hose reel jets and two main jets to extinguish the fire.
"The cause of the incident is thought to be accidental.
"Crews left the scene at 23.38pm."
The owner said the gym remains open and unnaffected by the fire.
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