The company, which runs 66 low-cost gyms across the UK, raised about £90m through the initial public offering (IPO) of its shares, which began trading at 195p each.
Shareholders including staff, senior management and private equity firms Phoenix Equity Partners and Bridge Ventures have raised a further £35m.
Chief executive John Treharne said: “Today marks a significant milestone in the development of The Gym and we are delighted at the strong levels of investor interest and support for our IPO.
“The Gym gives members the freedom and flexibility to improve their health and fitness on their own terms, removing the old barriers of fixed, high-cost contracts which have served to reduce accessibility in the past. We operate a successful, innovative and technology-led disruptive business model, driven by an experienced management team and highly motivated colleagues. We believe that these factors give us a significant competitive advantage and we look forward to using our listing to realise our growth strategy.”
The firm called off a planned £300m merger with rival Pure Gym last year due to the prospect of a “protracted” investigation by the UK’s competition watchdog.
In 2014, The Gym generated adjusted underlying pre-tax profits of £14.7m, on revenues of £45.5m.