The move follows a competitive pitch with a number of agencies.
Prosek Partners UK said a combination of digital and social media activities are planned to support earned and owned media campaigns to raise awareness of services from ACI, which has a market cap of $2.3bn.
Fred Duff Gordon, associate director at Prosek, said: "The global payments industry is evolving rapidly, with a major impact across retail banking, transaction banking and the retail sector. Innovation in payments technology is rapid. Our task is to demonstrate to banks, retailers, merchants and processors how ACI is taking a leading role in this changing payments landscape and ultimately how it can help them to get ahead."
Felicity O’Brien, head of marketing EMEA at ACI Worldwide, said: "With its strong focus on fintech and payments expertise, Prosek fits the bill as an insightful and creative team to help us deliver a complex message across all media channels. Their team presented a very ambitious but attainable PR plan and we look forward to seeing it fulfilled."
The London arm of Prosek Partners was founded in 2012 by Andrew Waterworth, who had previously spent 15 years at Financial Dynamics (later FTI Consulting).