Streptococcus agalactiae, group B Streptococcus (GBS), infects and causes severe diseases in humans and numerous animal species, including fish, given its ability to cross the host-specific barrier. The emergence of antibiotic resistant GBS strains makes it necessary to look for alternatives to treat and prevent infections that it produces. The aim of the present study was to determine the antibacterial activity of ethanolic and aqueous extracts of medicinal plants from Misiones province, northeast Argentina, against GBS from humans and fish. We used human Streptococcus agalactiae ATCC® BAA-611™ and tilapia Streptococcus agalactiae ATCC® 51487™ strains. Minimum Inhibitory Dose (MID) was determined by the disc diffusion method. Minimum Inhibitory Concentration (MIC), Minimum Bactericidal Concentration (MBC), MBC/MIC ratio, drug synergism with commercial antibiotics, and resistance assays were determined with extracts that showed antibacterial activity. Medium Lethal Dose 50 (LD50) was determined by the Artemia salina assay. For  ethanolic and aqueous Eugenia uniflora L. extracts, we obtained a MID = 0.5 mg.disc-1. For both extracts of Eugenia uniflora L., the MIC and MBC values were 1 mg.mL-1 and 5 mg.mL-1, respectively. The MICI (MBC/MIC ratio) = 5 qualified the action of these extracts as bacteriostatic. The drug synergism assay with ampicillin, erythromycin, and clindamycin combination and extracts showed indifference. The LD50 of the aqueous extract was 0.82 mg.mL-1 indicating moderate toxicity. This work is a first step to identify chemical compounds in native medicinal plants of Misiones, Argentina, that could mean an alternative for the treatment of Streptococcus agalactiae infections.