Marcus Wooldridge, 36, of Blandamour Way, Bristol, was found guilty at Bristol Crown Court and sentenced to two years and nine months.
The attack happened at the Fit Firm Gym in Hotwells Road - formerly owned and managed by Wooldridge.
Police believe there may be more victims as he worked in several gyms.
Investigating officer Det Con Rachel Alford said: "Wooldridge claimed to be a trained physiotherapist and registered nurse, which gave him the opportunity to physically examine the victim within the gym environment.
"The truth is he has no formal qualifications as a physiotherapist or personal trainer and these claims were merely a smokescreen for his real intentions - to commit a sexual assault."
After the sentencing, the victim who is now aged 20, said: "I'm so relieved this is over and I want to warn people that these types of crimes can happen to anyone.
"In my view it's worth speaking out about and I hope this conviction gives other victims the courage to report offences to the police."
Wooldridge was also placed on the sex offenders register for life.
A gym owner who sexually assaulted one of his clients under the guise of helping her with stretches and treatment has been jailed.