Challenging behavior exhibited by young children with neurodevelopmental disabilities is known to negatively affect their optimal development and families' quality of life. Although some support exists for English-speaking parents of children with autism who live in high-resource countries, it is scarce for others. Such disparity may also be heightened during the coronavirus disease 2019 (COVID-19) pandemic with its various societal restrictions. To mitigate the treatment gap, we developed and adapted a series of online training modules on behavioral principles and examined its effectiveness with 88 South Korean parents using a randomized controlled trial. We found significant interaction effects on increasing parents' knowledge of behavioral principles, increasing positive parenting practices, and decreasing parental stress. Qualitative social validity data also indicated that parents were highly satisfied with the goals, procedures, and outcomes and that the program positively affected their parenting styles.