Metra Seis will eventually be integrated with the existing Spanish business of Synovate, the market research division of Aegis, to create scale advantages and a broader client offer.
Metra Seis was founded in 1964 and currently has 75 staff in Madrid and Barcelona and has gross assets of €3.4m (£2.4m).
Aegis said that Metra Seis's expertise in quantitative research will fit well with Synovate Spain's existing position in qualitative offering.
Aegis also said that the Spanish agency's strong presence in the automotive and FMCG sector is "highly complementary" to Synovate's established practices and solutions in these areas.
Robert Lerwill, CEO of Aegis, said: "Metra Seis is a very good addition to Synovate Spain, creating a highly competitive new player. Our clients in Spain stand to benefit from the combination of Synovate's global tools and knowledge with Metra Seis's well-established local capabilities."