Ikeda Mitsumasa’s Shinto-related reforms in Okayama domain in the later 1660s have hitherto been interpreted as measures of local relevance. By applying the Domain Shinto paradigm to this case, however, it becomes clear that the reforms are local manifestations of a much broader appreciation of Shinto among daimyo of Tokugawa kin. Mitsumasa’s reforms are best known for the adoption of religious certification via Shinto shrines (shintō-uke) instead of Buddhist temples (terauke) as part of the practice of sectarian registration (shūmon aratame). In Okayama, this brought about a domain-wide separation of Shinto and Buddhism (shinbutsu bunri), a most radical measure that had to be abandoned under the regime of Mitsumasa’s successor Ikeda Tsunamasa. Nevertheless, this article demonstrates that Okayama’s Domain Shinto reforms brought about a lasting functional differentiation between Shinto and Buddhist clergy leading to a professional Shinto priesthood even at the level of village shrines. Thus, Okayama became a pioneer region in regard to the development of Shinto autonomy.