City Index claims to have ambitious growth plans for 2012 and has charged the agency with helping expand its customer base in its key markets.
It is understood that Total Media has been the incumbent on the account since 2009 when it won a competitive pitch against Gotham Financial Services.
Chris Armond, business director of Arena Media, said: "City Index is an exciting new client with big ambitions to grow their business.
"We believe our performance-based approach and their willingness to think differently is the perfect way to help them realise their ambitious targets."
Paul Cassidy, global head of marketing for City Index, said: "Arena Media demonstrated a deep understanding of our business and its challenges.
"We were hugely impressed with their data management and insightful approach."
As well as the UK account, Arena Media will service Poland, Germany, Singapore and Australia across offline and online channels including TV, press, outdoor, display and email.
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