Fitch and RPA's existing studios in Columbus, Ohio, will be merged to form Fitch:RPA, representing a significant consolidation in the fragmented retail and product design sectors.
Together the combined agency will have blue-chip clients including BP, Merrell, SC Johnson, Sprint and Toys 'R' Us.
RPA employs 53 people and had revenues of $8m (£4.5m) for the year ended December 31 2003 and net assets of $2m.
The acquisition of RPA reinforces WPP's comprehensive marketing services offer and continues the group's strategy of strengthening its practices in important markets and sectors.
The growth of Fitch in the US follows expansion in South East Asia for the firm, which was acquired by WPP when it took control of Cordiant Communications last year.
In February Fitch announced plans to grow its operations in China to capitalise on growing demand for retail branding and design services.
Fitch has an extensive presence in the region with offices in Singapore, Hong Kong and Sydney.
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