The survival and spreading of resistant bacterial strains has boosted research which focusses on discovering new antimicrobial agents derived from plant species. Several studies have demonstrated that substances present in plant extracts can modify the activity of antibiotics, increasing their efficacy. Species of the genus Psidium have been popularly used to treat bacterial infections. However, their modulatory effect on antibiotic activity remains to be elucidated. The aim of this study was to evaluate the modulatory effect of the hydroalcoholic extracts obtained from the leaves of Psidium guajava L. and Psidium brownianum Mart ex DC on bacterial proliferation, both separately and in combination with antibiotics. The assays were performed using the microdilution method. The minimum inhibitory concentration (MIC) of the extracts and drugs were determined against standard and clinical isolates of Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus. To verify the potentiation of the antibiotic activity, the MIC of the antibiotics were determined alone and in association with the extracts. The extracts of P. guajava and P. brownianum did not present clinically significant activity against the Gram negative bacteria evaluated, with MIC values against S. aureus of 256 and 512μg/mL, respectively. However, when combined at sub-inhibitory concentrations with antibiotics, both extracts presented significantly synergistic effects. Our results demonstrated the effectiveness of P. guajava and P. brownianum on modulating bacterial growth, suggesting that these natural products might be used in drug development in association with antibiotics, reducing bacterial resistance and thus, improving the treatment of bacterial infections.