Baillie Gifford has given a book of up to 21,000 investment trust clients to Hargreaves Lansdown for free.
The agreement covers clients who hold Isa, share plan and children's savings plan accounts that had been administered directly by Baillie Gifford, but will now become clients of Hargreaves Lansdown.
A total of £1.3bn of assets will move to the Hargreaves Lansdown platform.
Baillie Gifford has said no money or consideration changed hands as part of the agreement.
Under the terms of the agreement, the clients will pay the same to Hargreaves Lansdown as they had to Baillie Gifford, with the fee level frozen for a minimum of three years.
James Budden, director of retail marketing and distribution at Baillie Gifford, said: "The increasing variety, capability and cost effectiveness of investment platforms in the wider savings market has led us to decide plan holders of our investment trust savings scheme are best served by a specialist platform.
"We selected Hargreaves Lansdown for a number of reasons, including its ability to offer efficient access to our entire investment trust range through a broad selection of savings products."
Baillie Gifford confirmed the clients would continue to be able to place money into the funds during the period of the transfer.
Chris Hill, chief executive of Hargreaves Lansdown, said: “As one of the largest supporters of investment trusts we are pleased to welcome Baillie Gifford clients to Hargreaves Lansdown.”
This is the ninth such agreement reached between Hargreaves Lansdown and a fund providers.
Previous agreements have been reached with BlackRock, JP Morgan Asset Management, Jupiter, Legg Mason, Old Mutual Global Investors, RIT Capital Partners and Witan.
Clients are not obliged to switch from Baillie Gifford to Hargreaves Lansdown. Under the terms of the deal, they can switch to another platform of their choice but cannot remain with Baillie Gifford.