The present study ethnographically investigates language ideologies pervasive in Japanese young people’s English use on SNS, as reflected in their online practices and interview narratives. The study illuminates how the participants’ daily engagement with English on SNS is shaped by multiple ideological discourses embedded in Japanese society, including the perception of English as an asset, native speakerism, and prescriptivism in English. The data were collected through a 5 to 11-month ethnographic investigation of Instagram practices of 14 Japanese participants in their 20s, who have high English proficiency due to their returnee and sojourner experiences. A transtextual analysis (Pennycook, 2007) of the participants’ online posts and narratives reveals that they often described using English on SNS as their “pride”. Some considered using “authentic” and “perfect” English they acquired during their previous stays in English as a native language (ENL) countries as a means of distinguishing themselves from their Japanese friends who have grown up primarily in Japan and have comparatively lower English proficiency. To maintain their self-presentation as proficient English users, they showed a strong orientation toward the grammar correctness of their Instagram posts, considering that their English is subject to “online peer surveillance” (Trottoir, 2016) by their SNS followers, who occasionally correct their English. The participants’ attitudes underscore the entrenched perception of speaking English, along with the experience of studying in ENL countries, as an asset to advance their social status (Iino, 2021; Kubota, 2019). Additionally, the commitment to grammar accuracy and peer English correction unveils English prescriptivism, which can be traced back to the long-standing influence of English education that emphasizes examinations and grammatical competence (Murata, 2020). By investigating linguistic experiences on SNS, this study highlights the significance of critically examining the impact of ideologies and social values associated with English in Japan on young people’s everyday language use.