Summary Streptococcus mutans has been associated with the development of dental caries. Semidesert bush Jatropha dioica Sessé, found in northern Mexico and widely used as traditional medicine, is known to inhibit pathogenic bacteria. The Folin–Ciocalteu method and free radical capture by DPPH technique were used to determine total polyphenols (TP) and antioxidant activity (AA) respectively. The toxicity and antimicrobial activity of the extract with the highest TP and AA were determined. The highest value of TP (46.06 mg GAE g −1 ) was determined in the dried root extract. A significant and positive correlation was observed between TP and AA. In the flavonoid quantification, the extract showed the highest value of 4.57 mg QE g −1 . The hydroalcoholic extracts did not show toxicity. Hydroxycinnamic acid compounds and methoxyflavones and hydroxybenzoic acid were found in the extracts. These compounds are responsible for antibacterial activity. The minimum inhibitory concentration (MIC) against the Streptococcus mutans was found to be 3.75 mg mL −1 .