Background: The aim of this study was to detect and molecularly identify Rickettsia spp. in Rhipicephalus sanguineus (sensu lato) collected from free-roaming dogs in 30 communities from five municipalities in the south of Coahuila State, northern Mexico, where Rocky Mountain spotted fever is endemic. Methods: In total, 60 dogs from each municipality were examined for engorged ticks. DNA was isolated from tick pools and conventional PCR assays targeting the 23S-5S ribosomal RNA intergenic spacer and outer membrane protein (ompA) gene of Rickettsia spp. were performed. Results: All ticks (n = 1238) were morphologically identified as R. sanguineus (s.l.). Six pools (each with six engorged females) from four municipalities were positive to Rickettsia spp. DNA sequencing and phylogenetic analyses confirmed the presence of R. rickettsii and R. rhipicephali in R. sanguineus (s.l.) in these ticks. Conclusions: This study confirms the presence of R. rickettsii and R. rhipicephali in R. sanguineus (s.l.) from stray dogs in the south of Coahuila. This suggests that stray dogs may play a role in the inter-municipal dissemination of infected ticks in this region. Further research is required to assess whether ticks from stray dogs could serve as good indicators for the molecular xenomonitoring of R. rickettsii in this region. Considering that R. sanguineus (s.l.) is a proven vector of R. rickettsii in Mexico, increased awareness regarding permanent tick control in dogs is warranted.