A conditional placing to raise a net £25mln at 58p per share will fund the acquisition, with up to a further £1mln to come from an open offer.
FairFX said the placing was heavily oversubscribed by new and existing institutional shareholders with some £4.7mln of the funds coming from VCT/EIS investors.
CardOne provides a digital banking platform to small businesses as well as having 80,000 current account customers.
It is one of the few card providers able to offer a comprehensive digital banking service said FairFX, which will use CardOne Group's infrastructure and technical capability to speed up its own roll out of a small business banking product.
Business foreign exchange offers a bigger market opportunity than consumer cards where competition has increased, it said.
FairFX already believes it has a significant competitive advantage in the SME market through its business expense management platform.
The CardOne acquisition is expected to be earnings enhancing in the first full year following completion, with cost savings and synergies estimated at £3.5mln over 3 years.
CardOne made a profit of £800,000 on sales of £5.5mln in 2016 and posted a profit of £600,000 in the first half of 2017.
Ian Strafford-Taylor, FairFX’s chief executive, said CardOne represented a major step forward for the company.
“To date, FairFX has been successful in disrupting the currency payments market by offering its customers a better service and value. CardOne has done much the same in the banking space, and together we believe we can provide a product offering which will deliver greater services and faster growth."