In this article, I explore some of the ways that the Heart Sutra is used by mountain ascetics in Dewa Sanzan, a sacred mountain range in Yamagata Prefecture, Japan, where I conducted twenty-four months of intermittent ethnographic fieldwork between 2012–2019. Dewa Sanzan is comprised of Mount Haguro, Mount Gassan, and Mount Yudono. There is a longstanding historical conflict between Buddhist-oriented and Shintō-oriented institutions of Shugendō in Dewa Sanzan that is especially concentrated in Mount Haguro. This rivalry stems from the shinbutsu bunri imperial edict in the early Meiji period (1868–1912) in which syncretic religions such as Shugendō were forced to purge Buddhist elements. In Dewa Sanzan, this process led to the reinvention of Shugendō as a Shintō-oriented religion. Buddhist Shugendō was, at the time, persecuted and greatly reduced from its former state. However, Buddhist Shugendō was revived in the postwar period when religious freedoms were expanded in Japan. The establishment of Shintō-oriented Shugendō in the Meiji period and the revival of Buddhist Shugendō in the postwar period has led to a situation in which the two institutions of Shugendō share the same contested mountainous space. This article considers the role of the Heart Sutra in negotiating social, religious, and political boundaries between ascetic rivals.