The retailer claimed it would “revolutionise the market” with plans to open 200 sites charging a fraction of rivals’ prices.
A flagship 20,000 sq ft site is to open at Aintree on Merseyside next month alongside a new Sports Direct store and it is charging just £5 a month in membership fees, on top of a £10 joining fee.
Another gym will follow at Keighley, West Yorkshire.
A spokesman for Sports Direct said: “Our gym memberships will offer fantastic value for money in the same way that our customers have come to expect from our retail outlets and our website.
“This will be affordable fitness on an unprecedented scale. It will revolutionise the market by making private gym membership a realistic lifestyle choice for millions of people.”
The announcement comes after a recent deal to acquire up to 30 sites from LA Fitness, which is in the process of rebranding.
Sports Direct said its price deal was being trialled at its new purpose-built gyms with a view to rolling it out at the former LA Fitness sites in the future.
It is the latest venture for the group, which operates around 418 stores in the UK and has seen a rapid rise to become a FTSE 100 company since being founded by Mr Ashley in Maidenhead in 1982.
The group, still controlled by Mr Ashley, has recently taken a major stake in department store Debenhams and placed a multi million-pound corporate bet on shares in Tesco. Mr Ashley has also acquired an 11% chunk of House of Fraser.
Meanwhile, he has become involved in a football club boardroom battle over the control of Glasgow Rangers.
Mr Ashley’s move into cut-price gyms comes months after two operators - Pure Gym and the Gym Group - abandoned plans for a merger that would have created a new low-cost player with more than 100 sites.
They ditched the merger in July in the face of a full-scale competition investigation which would have lasted until next year,