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In this paper, a very small 4-port MIMO antenna is designed, based on a metamaterial structure composed of embedded octagonal Split-Ring Resonators (SSRs). The antenna array shows an axial symmetry configuration with dimensions of 32× 32 mm2, corresponding to 0.157λ2, approximately, related to a center frequency of 3.5 GHz, with a high electromagnetic isolation despite the radiators’ closeness, reaching values bigger than 26 dB for adjacent antennas, and more than 28 dB for opposite antennas. The antenna is built on a substrate with dielectric permittivity of 2.2 and 1.27 mm thick. The Total Active Reflection Coefficient (TARC) presents a steady behavior for different random phases at the incoming signals, keeping a system bandwidth of 0.9 GHz for a− 10 dB value. On the other hand, the Envelope Correlations Coefficient (ECC) reports values lower than 0.001 in all the antenna relationships, achieving a very uncorrelated performance of the electric fields in each element. The radiation pattern is quasi-omnidirectional, obtaining a low gain around− 2 dBi, a trade-off that is considering the size reduction of the MIMO antenna.