Background: Priority given to supported decision-making has increased internationally since the United Nations General Assembly adopted the Convention on the Rights of Persons with Disabilities (CRPDs). Nations worldwide have moved to ratify and enact the CRPD. Methods: A questionnaire survey on the supported decision-making status was conducted in Japan and Taiwan (60 and 73 people, respectively; N = 133, 82 women and 51 men). Survey items were objective indicators based on the CRPD. Results: In both countries, people who are older (vs younger) or those having more (vs less) experience supporting persons with intellectual disabilities were more likely to practice supported decision-making. Relationships between persons with intellectual disabilities and family members are vital. Conclusions: It is necessary to establish appropriate supported decision-making methods, that do not depend on aspects such as support experience, for persons with intellectual disabilities in Japan and Taiwan. Additionally, it is necessary to clarify the relationship with the family in supported decision-making for persons with intellectual disabilities and to work together appropriately.