Digithais will become part of Isobar, Aegis' fast-growing digital network, and follows on from its purchase in January of Polish interactive agency Hypermedia for £5m.
In January, Aegis also acquired Italian market research firm AMT Consulting for £3.2m to become part of its global Synovate network.
The purchase price for Digithais, which was founded in 1996, is subject to "rigorous performance criteria".
Digithais has a blue-chip client roster of local and international companies including Avon Cosmetics, BMW, Heineken and Philips Electronics.
The agency provides clients with website development, internet marketing, content management and e-commerce services.
Robert Lerwill, CEO of Aegis Group, said: "Aegis is rapidly building a global interactive network, servicing the needs of some of the world's best known marketers, and Digithair now provides us with strong presence in Thailand."
The acquisition follows yesterday's announcement that the media and communications market research group, and owner of Carat, is on course to hit targets after success at Synovate and an increase in US marketing spend.
Aegis plans to report its first half-results on September 6, for the first time using International Financial Reporting Standards. Its first-quarter results, which were revealed in May, showed a rise in revenues of 12.5%.
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