The Meapa Gymnastics Club in Cedar Avenue, Gravesend, was targeted at around 12.45am on Monday.
A man broke down the door to the club’s office and stole around £400 from the till.
He was captured on CCTV.
Coaching director Yuriy Zhovtyuk said: “It doesn’t seem like a lot of money but we’re a charity organisation and it’s a lot to us, especially when somebody stole another £500 not long ago.”
The gymnastics centre had already been broken into at the end of May.
The management committee installed more alarms and motion sensors, but it has not put burglars off.
Mr Zhovtyuk continued: “The morning staff are now very worried about what they could find when they get into work and our cleaner is frightened to work on her own in the evening in case anybody decides to break in again.
“I think it must be the same people – they knew where to go, and where to find the money. The door was pulled completely off its hinges but last time they did far more damage, smashing windows and pulling locks off the cupboards to hunt inside.”
Meapa is a non-profit organisation, with whatever money it raises going straight back into supporting the club’s facilities and improving them for more people to use.
It is one of the oldest clubs in Kent and South East England, having been established in 1976.
It has approximately 700 members, ranging from six months to adults, and was selected as a training facility for the 2012 Olympic Games.
The 36-year-old added: “We are trying to get young people more involved in being active and we have a lot of equipment here. We’re all worried that could be taken next time.”
A forensic team arrived at the club on Monday morning to look for fingerprints and the door has been secured temporarily.
The club is now considering installing metal doors to stop people breaking them down, but for three it will cost around £4,000.
Some fundraising events are being planned to improve the club’s security.