Champneys will finalise the deal for the historic hotel on November 1, when it will be renamed as Champneys at Eastwell.
Eastwell has 62 guestrooms and sits within 3,000 acres in the Kent countryside. It is now set to undergo a refurbishment programme which will be completed by next summer.
Champneys was established in 1925 and now operates four resort spas and six city spas in the UK.
Owner Stephen Purdew said: “The acquisition of Eastwell Manor is a major milestone for the group. As the original pioneers of health spa resorts in the UK, we are delighted to be welcoming such a landmark property and for Eastwell Manor to develop its own wellness credentials.”
The move marks the latest phase of a major development programme for the group, which has also seen the refurbishment of Champneys Tring and Champneys Forest Mere.
Further refurbishment is also planned for Champneys Henlow and Champneys Springs.
Purdew said: “We are continuing to look at our future growth and with recent investments in our signature properties we are very excited about wellbeing in the UK.”