Full Circle Media, which provides media planning and buying services, has a client list that includes Goodyear, Media24 and Safmarine, and reported gross assets of £0.4m.
Aegis said the acquisition would help strengthen its client presence in South Africa, and complements its existing media agency portfolio in the country, which includes MEC Carat.
The group also launched digital marketing specialist Trigger in South Africa in May and unveiled Vizeum South Africa in September.
The acquisition continues a busy period of international expansion for Aegis, having acquired Australian promotional marketing agency Apollo at the beginning of the month, Canadian agencies Genesis Media and Mindblossom in October and November respectively, and French promotional marketing and direct agency Implicom also in October.
Robert Lerwill, chief executive officer of Aegis, said: "Full Circle Media is a well-run agency, and will fit well with Aegis Media's existing business in South Africa.
"We see an attractive opportunity to build up a leading position in a dynamic economy undergoing rapid change, and we are acting on it with three major investments here this year."