The sale, for an undisclosed fee, comprised Thesis’ private client business and its Financial Planning arm, Pallant Independent.
The business development team and the four private client teams will move to the Sanlam wealth management business.
They will continue to operate from current locations in Brighton, Chichester, Guildford and Lymington.
David Tyerman, managing director of Thesis, said: “We have sought to find an acquirer that matches the Thesis culture of client focus and professionalism. In Sanlam we believe clients, staff and business partners will find that this culture is embedded into the group.
“Sanlam Limited is an international financial services group with a heritage dating back to 1918.
“Globally, the business employs over 17,000 people and manages $70 billion of client assets.
“In the UK it has a network of 13 regional offices and employs 450 professionals.
“The larger group brings scope for far greater investment into an enhanced investment proposition and client service, together with improved asset growth and career progression opportunities for employees.
“This is a good outcome for all of our clients because it combines continuity of service locally whilst introducing in the background a greater level of systems capability, research resources and an ongoing investment plan to ensure that the business adapts rapidly to all the challenges that will undoubtedly arise.
He added: “Sanlam’s product offering is truly international, and managers and clients will be able to invest globally
“The client facing teams are remaining intact and will continue to provide service and advice as before, from the current office locations
“There will be no change to personnel, service or fees.
“All clients will be introduced to an enhanced proposition once the business is fully integrated.”
Jonathan Polin, Sanlam UK chief executive said: “Thesis is a high quality business and an important strategic fit for Sanlam in the UK.
“It was clear from the outset of our discussions with the Thesis management team that we were fully aligned in terms of investment philosophy, culture and an unwavering commitment to client service.
“We have high ambitions to make Sanlam a leader in client-centric wealth management and our acquisition of Thesis is an important milestone in that journey.
“The entire Sanlam family is looking forward to welcoming our new colleagues and working together to meet the financial needs of our clients.”
Thesis was described as having a significant presence in the South of England, with £1.2bn under management and a team of 30 investment professionals and their support colleagues.
The deal will take Sanlam’s UK private client discretionary assets under management to £4.2bn.
Following regulatory approval Thesis will operate under the Sanlam brand, it was confirmed.