Sanctuary's spa in Covent Garden, Central London, attracts 64,000 guests a year and it has an exclusive deal with Boots to sell its products on the high street.
Cussons plans to open more day spas in the UK and overseas, as well as extending the range of Sanctuary-branded health and beauty products, which include mud wraps and exfoliating salt scrubs.
Cussons bought the business from HgCapital, which has owned the spa for the past 15 years.
Alice Alvis, who left Marks & Spencer to become the chief executive of Sanctuary three years ago, is understood to have earned £5m from the deal. She will remain with the business and oversee its expansion, having already begun a push into the US and the Far East.
The deal marks the latest move by Cussons to acquire more brands in the UK. The company bought the Charles Worthington haircare brand three years ago, following its acquisition of shower and bath products brand Original Source in 2002.
The spa gained notoriety in 1978 when Joan Collins appeared in a scene on the Sanctuary's swing over the pool in the film 'The Stud'.
It was also popular with celebrities including Kate Moss and Sadie Frost in the 1990s.