Leishamaniasis is a disease that affects more than 2 million people worldwide, whose causative agent is Leishmania spp. The current therapy for leishmaniasis is far from satisfactory. All available drugs, including pentavalent antimony, require parenteral administration and are potentially toxic. Moreover, an increase in clinical resistance to these drugs has been reported. In this scenario, plant essential oils used traditionally in folk medicine are emerging as alternative sources for chemotherapeutic compounds. In this study, in vitro leishmanicidal effects of a thymol- and a carvacrol-rich essential oil from leaves of Lippia sidoides Cham. were investigated. The essential oils were extracted and their constituents were characterized by gas chromatography coupled to mass spectrometry (GC/MS). Both essential oils showed significant activity against promastigote forms of Leishmania chagasi. However, we found that carvacrol-rich essential oil was more effective, with IC50/72 h of 54.8 μg/mL compared to 74.1 μg/mL for thymol-rich oil. Carvacrol also showed lower IC50 than thymol. Our data suggest that L. sidoides essential oils are indeed promising sources of leishmanicidal compounds.