Aegis is acquiring 100% of the firm in a deal that will see it pay an initial £4.8m, with a further deferred performance-related payment made over three years of up to £14m.
The five-year-old agency, which has offices in Atlanta, Chicago, San Francisco, Washington DC and London, specialises in sports and entertainment sponsorship and lifestyle marketing.
Doug Flynn, CEO of Aegis Group, said: "The acquisition of Velocity is consistent with our strategy of investing in the development of strong global networks.
"Sponsorship has become a key communication medium and a vital part of the communications planning mix. We were very impressed with the vision of Velocity and are keen to develop this part of the business into a true international capability."
Velocity is ranked fifth by US trade magazine Promo in its annual ranking of sports and entertainment marketing agencies.
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