Syzygy, which has offices in London, Frankfurt and Hamburg, says it is planning further acquisitions in the future.
Founded in 1999 by two Germans, Florian Schmitt and Alexandra Jugovic, Hi-ReS! has a string of awards to its name, including a Bafta, Cyber Lion and a Clio. Its clients include Nokia, Sony, EMI, Adidas, Dolce & Gabbana and Diesel.
Marco Seiler, Syzygy's CEO, said: "Hi-ReS! is one of the world's first stops for digital marketing solutions. With its acquisition, the Syzygy Group enlarges and further strengthens its team of top creatives. More than ever before, we are now able to assume idea leadership for top international brands."
The agency, whose names stands for "Handsome Information - Radical Entertainment Systems", will continue to run independently within the Syzygy Group.
Gareth Phillips, managing director of Syzygy in London, said: "We are very much looking forward to working alongside the Hi-Res! team, there are some real synergies to be had that will offer clients added value."
Last year Syzygy acquired London-based Unique Digital and GFEH in Hamburg, Germany.
Syzygy now has more than 250 staff and generated sales of more than £18.5m in 2007.