This study describes the cross-cultural adaptation to Brazilian Portuguese of the Adaptive Behavior Assessment System (ABAS-3), which assesses 11 skills areas within three major adaptive domains: the conceptual, social, and practical. The translation was performed by two independent translators, which was followed by the synthesis of the translations by two experts and then an analysis of the synthesis by 33 specialists from the areas of health and education (three per subscale) who were experts in the domains evaluated by ABAS-3. Of the 1121 items of the five ABAS-3 forms, 82 (7.31%) needed revision. There was good agreement between the specialists in respect of most items, with only minor adjustments being required. There was good evidence of content validity and the adequacy of the adaptation in respect of the conceptual, idiomatic, and semantic aspects of the items of the instrument. This initial process of cross-cultural adaptation was necessary because adaptive behavior is strongly influenced by environmental factors such as socioeconomic status and culture.