This data donation study explores how adolescents express emotions on social media, and how this expression varies by platform (Instagram, Snapchat, and TikTok) and posting modality (permanent and ephemeral content). Adolescents from Belgium (n = 115), France (n = 146), and Slovenia (n = 119) “donated” screenshots from their social media accounts, including their bios, most recent posts, and most recent stories. These screenshots were coded for both textual references to positive and negative emotion words and emojis, and visual expressions of positive and negative emotion. This data was then linked to the adolescents’ survey responses about their life satisfaction. The results showed a prevalence of positive emotional displays, both in text and visuals. Gender differences were evident, with girls more likely than boys to reference and express positive emotions. Additionally, a marginal difference was observed among countries: Slovenian participants showed fewer positive emotional references compared to Belgian and French participants. No significant links were found between online emotional expressions and life satisfaction. While this study offers initial insights into adolescents’ real online emotional behavior, future research should explore motivations and sociocultural influences, addressing gaps in understanding how individual and contextual factors impact social media use among adolescents.