PURPOSE: Autistic children and their families from heritage-language-speaking homes are underrepresented in empirical research and would benefit from receiving care tailored to their linguistic and cultural needs. Board Certified Behavior Analysts (BCBAs) commonly support autistic children in the United States but have reported limited knowledge and training in how to support bilingual children, their families, nor the bilingual staff who support them. The following research question was addressed: Do BCBA leaders in autism care who complete an asynchronous online professional development training demonstrate (a) increased knowledge, (b) improved self-efficacy, and (c) improved attitudes toward supporting multilingual Applied Behavior Analysis (ABA) staff and recipients of multilingual ABA services compared to BCBA leaders in a waitlist-control group? METHOD: Part of a larger sequential exploratory mixed methods study, this randomized waitlist control trial measured the impacts of a professional development training related to autism and multilingualism in ABA care on BCBAs' knowledge, self-efficacy, and attitudes. Social validity data were also collected and analyzed using descriptive statistics. RESULTS: The training significantly increased the intervention group's knowledge and reported self-efficacy compared to participants in the waitlist control group but had no significant effect on participants' attitudes. In addition, participants perceived the training as socially valid. CONCLUSION: Implications highlight the need for more attention on professional development research for autism providers, to examine pre-service training, and for clinical leaders to examine their workplace environment to promote effective and fair practices.