Sophron has 45 consultants mainly based in its London office, although it also has an office in Cologne.
It has annual turnover of around £6m, almost half of which comes from its business in mainland Europe, and its clients include Vodafone, Orange, Sky and MBNA.
CACI has made the acquisition to strengthen its marketing services division.
Greg Bradford, chief executive of CACI, said: "Sophron provides skills and experience that complement CACI's expertise in consumer and market analysis. This enables us to expand our presence in an area where we believe there are significant future opportunities."
Terms of the deal were not disclosed. Sophron will move from its offices in London to those of CACI.
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