Aim: The study aimed to compare the in vitro antibacterial activity of an ethanol extract of Rosmarinus officinalis 'romero' (EERO) at different concentrations with Streptococcus mutans ATCC 25175. Materials and Methods: We worked with three concentrations of 25%, 50%, and 75% of the EERO and compared these with the positive control chlorhexidine 0.12% and distilled water as a negative control. Seeding was performed in Müller-Hinton agar medium. The inhibitory effectiveness of EERO was determined using the disc diffusion method with the experimental solutions. The seeded and inoculated plates were incubated at 37°C, for 24 and 48 h. The reading was carried out according to the Kirby-Bauer method by measuring the diameter (mm) of the inhibition halo formed by the bacteria using a Vernier caliper and recorded on a data collection sheet. Results: In the 24-h group, chlorhexidine 0.12% showed the highest inhibition halos with a mean of 12.8 ± 0.8 mm, followed by EERO 75% with a similar mean of 12.1 ± 0.6 mm. Similarly, at 48 h, chlorhexidine 0.12% had the highest antibacterial activity of 13.0 ± 0.7 mm followed by EERO 75% with 12.2 ± 0.5 mm. Conclusions: The EERO showed inhibitory effectiveness against S. mutans, with the concentration of 75 mg/ml showing similar results, albeit not statistically significantly different, to those of chlorhexidine.