Selenium is an essential micronutrient and as an integral part of selenoproteins in the body it is essential for defense against oxidative stress. Nanoselenium has the best efficiency/safety ratio, but due to its tendency to agglomerate, it needs to be stabilized by coating during synthesis. In this paper, we investigated the possibility of using olive pomace extract (OPE) as a coating agent in the synthesis of nanoselenium. OPE was used at two concentration levels, 0.3 % and 0.6 %, and the characteristics of the obtained nanoparticles were compared with those obtained by a standard chemical synthesis. The characterization of obtained nanoparticles included determination of particle size distribution, zeta potential, antiradical (ORAC and TEAC method) and reductive capacity (Folin-Ciocalteu method). Obtained results showed that the application of 0.3 % OPE in biogenic synthesis of nanoselenium results in a particle size distribution between 100 nm - 200 nm, which is similar to the particle size obtained by the standard chemical synthesis process. The increase of OPE concentration in reaction mixture significantly increased the mean particle diameter and the range of the particle size distribution. Zeta potential of biologically synthesized nanoparticles showed significantly higher absolute values (-34 mV) compared to chemical nanoselenium (-17.2 mV). Based on obtained data, it can be assumed that, unlike polyvinylpyrrolidone used in chemical synthesis, OPE stabilizes the nanoparticles primarily by electrostatic interactions. Unlike chemically synthesized nanoparticles, biologically synthesized nanoselenium showed significant antiradical (TEAC, ORAC) and reductive (Folin-Ciocalteu) efficacy (125 mg TE/L; 281 mg TE/L and 200 mg GAE/L, respectively).