The deal, which is subject to regulatory approval, continues the marketing services group's strategy of buying stakes in overseas firms even in tougher economic times.
The acquisition is also consistent with WPP's plan to offset the slow rate of growth in western markets with new revenue streams in emerging markets.
The company provides field marketing, market intelligence, customer contact and brand activation services to some of South Africa's biggest companies and brands including Parmalat, Pioneer Foods, Tiger Brands and Unilever.
Smollan Group, which is based in Johannesburg, also operates in India and other African countries and has strategic alliances in Europe, North and Central America, Asia and Australasia.
It employs more than 12,000 people and reported combined revenues for the year ended February 28 2008 of ZAR 847m (£47m), with gross assets of ZAR 284m (£15.9m).