This study sought to identify the knowledge of early childhood education teachers about Autism Spectrum Disorder (ASD) and Evidence-Based Practices (EBP) and their relationship to teachers self-efficacy beliefs about working with children with ASD. The research participants were 76 regular and adjunct teachers who work in child development spaces and municipal daycare centers in Rio de Janeiro. It is a quantitative-qualitative study in which, in addition to the participant characterization questionnaire, we used the Likert-type Self-Efficacy Scale for Teachers of Students with Autism (ASSET) and the questionnaire on ASD and EBP, with open and closed questions. The instruments were made available via online form, through the Google Forms platform, for the teachers to answer in a single moment. The findings were analyzed in two stages: coding of the open-ended responses of the ASSET and EBP questionnaire to generate raw scores of the teachers scores; and statistical analyses of the raw scores of the ASSET scale and knowledge about ASSET and EBP. They were discussed in light of the theoretical framework presenting teachers knowledge about autism, PBE, and teachers sense of self-efficacy. In general, the results regarding knowledge about ASD indicated that our sample had partial and limited knowledge about the aspects involving the definition and characteristics of the disorder. Concerning the awareness about EBP, the results pointed to the frequent use of standard practices in the routine of the early childhood education classroom, and although some participants cited practices that fit into EBP and reported knowing the practices mentioned in the closed questions, the knowledge about EBP was low in our sample. Concerning the sense of self-efficacy and knowledge about ASD, the results indicated no correlation between them; that is, the level of theoretical knowledge about ASD is not related to how much the teacher feels able to deal with these students. In return, there is a positive and weak correlation between self-efficacy and knowledge about EBP, in which it was observed that teachers who felt more confident, with more self-efficacy beliefs, tended to know practices with scientific evidence. Future studies that seek to verify the existence of a correlation between what teachers claimed to be able to do with students with ASD and what is present in their practices may expand the findings of this research. In addition, more robust instruments should be used to verify the sources involved in self-efficacy beliefs and to identify how much effort and commitment teachers demand to make their inclusive practices with these students happen. Finally, we believe that one of the contributions of our work is to emphasize the scarcity of robust instruments to identify teachers knowledge about ASD and EBP and to point out the importance of thinking about in-service training programs for teachers who work in early childhood education schools, focusing on theoretical and practical knowledge about ASD, involving sources of self-efficacy. In addition, it highlights the urgency of public policies that support and guide teachers regarding EBP.