Little is known about which secondary metabolites are responsible for inhibiting pathogenic bacteria and reducing the pro-oxidant effect on the leaves of four medicinal plants used as phytobiotic in animal production. The aim of this study was to evaluate the antimicrobial and antioxidant activity of four medicinal plants (Anacardium occidentale, Psidium guajava, Morinda citrifolia and Moringa oleifera.) in vitro. A total of six bacterial strains were inoculated, then minimum bactericidal concentration (MBC) was evaluated in fine powder and minimum inhibitory concentration (MIC) and MBC were determined on the aqueous extract. Also, the in vitro antioxidant activity was evaluated through 1,1-diphenyl-2-picryl-hydrazyl, as well as the main secondary metabolites were identified and quantified by chromatographic analysis. The results showed that Anacardium occidentale and Psidium guajava leaves had higher antimicrobial activity against all bacterial strains. In addition, Morinda citrifolia inhibited S. aureus in the aqueous extract, although without in vitro bactericidal effect, while Moringa oleifera leaf did not show antimicrobial effect. All plants showed antioxidant capacity, standing out Anacardium occidentale and Psidium guajava. Mainly the leaves of Anacardium occidentale showed high concentrations of quercetin 3-O-glucoside-7-O-rhamnoside, kaempeferol-7-O-glucoside, quercetin, caffeic acid, and cinnamic acid. Apparently, the antimicrobial and antioxidant activity are due to the main polyphenolic compounds identified in medicinal plants (mainly Anacardium occidentale and Psidium guajava); however, further studies are necessary to elucidate the exact mechanism.