BACKGROUND: Internet-delivered cognitive therapy for social anxiety disorder (iCT-SAD), which is a therapist-guided modular web-based treatment, has shown strong efficacy and acceptability in English-language randomized controlled trials in the United Kingdom and Hong Kong. However, it is not yet known whether iCT-SAD can retain its efficacy following linguistic translation and cultural adaptation of treatment contents and implementation in other countries such as Japan. OBJECTIVE: This study aimed to examine the preliminary efficacy and acceptability of the translated and culturally adapted iCT-SAD in Japanese clinical settings. METHODS: This multicenter, single-arm trial recruited 15 participants with social anxiety disorder. At the time of recruitment, participants were receiving usual psychiatric care but had not shown improvement in their social anxiety and required additional treatment. iCT-SAD was provided in combination with usual psychiatric care for 14 weeks (treatment phase) and for a subsequent 3-month follow-up phase that included up to 3 booster sessions. The primary outcome measure was the self-report version of the Liebowitz Social Anxiety Scale. The secondary outcome measures examined social anxiety-related psychological processes, taijin kyofusho (the fear of offending others), depression, generalized anxiety, and general functioning. The assessment points for the outcome measures were baseline (week 0), midtreatment (week 8), posttreatment (week 15; primary assessment point), and follow-up (week 26). Acceptability was measured using the dropout rate from the treatment, the level of engagement with the program (the rate of module completion), and participants' feedback about their experience with the iCT-SAD. RESULTS: Evaluation of the outcome measures data showed that iCT-SAD led to significant improvements in social anxiety symptoms during the treatment phase (P