Everyone Active is offering over-65s a free gym or swim session at its centres across Southwark throughout October.
The council-funded leisure centres say more than three quarters of older people are not doing the recommended amount of exercise.
A survey of more than 1,000 OAPs showed 78% of them are not doing the level of activity they should in order to stay fit and healthy into old age.
The NHS recommends 150 minutes of aerobic exercise and two sessions of strength exercise a week for people aged 65 and over.
The same age group is also far less likely to regularly go to the gym – with just eight per cent saying they make regular trips.
“It’s our aim to create a friendly, welcoming environment that motivates people of all ages to get active,” said Mark Basker, Everyone Active’s regional contract manager.
“I would urge older people to take the opportunity to visit our centre free of charge and discover the many ways to enjoy being active.”