The Detroit-based company, which was founded in 1999 and counts Coca-Cola and Microsoft among its clients, will be rebranded "HelloWorld, a Merkle Company." Chief executive Peter DeNunzio will continue to lead the 370-strong business.
HelloWorld offers both technology and marketing strategy services. As well as Detroit, it has offices in New York, Los Angeles, Chicago and Seattle.
HelloWorld and Merkle are among the 13 loyalty providers "that matter most", according to a report last August from Forrester.
Michael Hemsey, executive vice president of Merkle Loyalty Solutions, said: "The acquisition of HelloWorld increases our collective scale in providing loyalty strategy, technology, and engagement services that support the life cycle of our clients’ programs, across industries.
"Bringing together two recent Forrester Loyalty Wave strong performers makes us one of the most significant and strategic players in our industry. We will continue to expand our global client roster, working with brands that leverage the sophistication of our people-based loyalty marketing and technology solutions."
DeNunzio added: "Adding HelloWorld’s expertise in promotion and loyalty to Merkle’s comprehensive suite of solutions, our combined organization offers clients world-class capability, from acquisition to retention to ultimate loyalty in the brand-consumer ecosystem."
The deal follows Merkle's acquisition last month of B2B media and marketing agency DWA.