AdWatch, which was founded in 2000, is headquartered in Moscow and has gross assets of €9.5m (£7.5m).
The agency offers strategic planning, online media planning and buying, creative services, website design and build, online PR and below-the-line services.
AdWatch, which works for a number of Russian and multinational brands, including Microsoft, VTB Bank and Canon, will become part of Aegis' digital network Isobar.
Robert Lerwill, chief executive of Aegis, said: "The recent growth of Russia has made Europe's 'Big Five' markets a 'Big Six'. It has the largest population and economy of Central and Eastern Europe, one of our key growth regions.
"AdWatch brings us a significant competitive advantage in a market where demand for digital services is outstripping supply. It will enable us to boost our services in Russia rapidly, bringing an experienced and excellent team on the ground to make Isobar the market leader."
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