<p>Social anxiety symptoms are highly prevalent in adolescents and negatively impact their social and academic functioning, highlighting the need for effective low-threshold interventions. This randomised controlled trial evaluated the guided online intervention SOPHIE for adolescents (<i>N</i> = 133; 11–17 years) with social anxiety disorder (SAD; treatment) or subclinical social anxiety (indicated prevention) compared to care-as-usual control condition and qualitatively explored their experiences. Outcomes were assessed at baseline, mid-intervention (4 weeks), post-intervention (8 weeks) and 5-month follow-up and analysed using linear mixed-effects models. SOPHIE did not significantly reduce social anxiety symptoms post-intervention but showed a significant between-group effect at follow-up (<i>d</i> = 0.67, 95%CI [0.32;1.02]). Subgroup analyses by diagnostic condition showed a significant between-group effect at follow-up in the subclinical social anxiety (<i>d</i> = 1.53, 95%CI [1.74;0.41]) but not in the SAD condition. Social functioning significantly improved at post-intervention and follow-up, with medium to large effects (post: <i>d</i>=-0.73, 95%CI [-1.08; -0.37]; follow-up: <i>d=</i>-0.32, 95%CI [-0.66; 0.02]). Qualitative interviews post-intervention revealed that participants found the intervention beneficial, although some found exposure exercises challenging and desired additional support. Very heterogeneous needs emerged regarding the guidance provided during the programme. Low-threshold online interventions for adolescents with social anxiety may be effective, particularly as an indicated prevention approach.</p>