The brand, known for its flexible opening times and lack of contracts, has taken over the former LA Fitness site.
It will be the only venue where Canary Wharf workers can train throughout the day and night.
A spokesman revealed PureGym was pumping nearly £1million into the project and said existing memberships would be automatically transferred when the facility re-opens.
The gym will be closed during the conversion, ready for launch at the beginning of December.
The spokesman said: “We focus our investment on creating a high quality fitness and activity environment that meets the needs of most people with cardio, weight training, resistance training, functional training and over 50 instructor lead classes per week.
“Newly converted sites have experienced a 70% increase in membership and this gives us confidence that this model is working.”
Changes to the LA Fitness layout include an extended gym area with more than 60 extra pieces of cardio kit.
The designated space for functional training will stay, while 15 adjustable benches will be installed in the free weights area.
Personal trainers will lead spin classes in a refurbished studio while other free sessions include aerobic, pump and step.
Fans of the PrimalSeriesTM workouts, including Strength, Core, Renew, Perform and Condition are in luck, as these will remain on the timetable.
Membership prices will be £34.99 for a no contract membership, which includes access to more than 100 PureGyms throughout the UK.