Embark says the deal, for an undisclosed amount, will increase its presence in the UK investment savings sector.
The transaction will bring around £6bn of assets under administration along with 30,000 new clients to Embark.
It will take the total size of the Embark Group to over £22bn in assets on behalf of more than 180,000 consumer clients.
Embark says it will also gain the benefit of a “highly skilled specialist” team.
The deal also includes the purchase of the Alliance Trust Savings building in Dundee, which will become Embark’s platform operation centre and group headquarters, with platform service partner FNZ also occupying space in the building.
The firm says the acquisition “will cement Embark’s position as one of the few long-term players in the UK retirement space able to offer a full range of capabilities suitable for all levels of client affluence and distribution channels”.
It follows a recent material capital raise which included funding from new investors; BlackRock, Legg Mason and Merian Chrysalis.
Phil Smith, Embark Group chief executive, said: “This acquisition presents a perfect opportunity to accelerate our growth strategy by acquiring a book of platform assets with high SIPP penetration, as well as complementary distribution relationships.
“This takes our group AuA to well in excess of £22bn and reinforces our position as one of the fastest growing disruptors in the UK investment savings and platform market.
“It also offers an excellent opportunity to establish a centre for our operations and growth in Scotland, enabling us to access local talent and fintech capabilities, assisted by cross-government support.”
Richard Wilson, chief executive of Interactive Investor, said: “I am delighted with this outcome. It is a good result for all concerned.
“Embark is the right partner to take the advisory and partnership business forward, with Dundee remaining an important platform and employment centre.
“The ATS jobs have been protected which was a key consideration and I have no doubt they will take the business from strength to strength.
“The divestment will allow ii to focus on its single purpose: to serve the self-directed retail investor and the increased scale means we can continue to invest in providing a class leading customer experience, excellent value and a wealth of impartial information to help customers make better-informed investment decisions.”