Social mindfulness entails the consideration of the interests of others while respecting their autonomy. Although long-term group identity generates in-group favoritism in social mindfulness decisions, the effect of temporary group identity on social mindfulness remains to be validated. This study examined this effect by recruiting a convenience sample of 74 adolescents from a youth basketball club, who were randomly assigned to either an in-group or out-group decision-making condition. A basketball game scenario was used to establish temporary group identities, referencing the Minimal Group paradigm for grouping while applying the SoMi paradigm in a real-world context. The results showed that adolescents exhibited higher social mindfulness (χ2 = 22.774, df = 1, p < 0.001) and preference-adjusted social mindfulness (χ2 = 6.861, df = 1, p < 0.01) toward in-group compared to out-group members. Conversely, they displayed higher levels of preference-adjusted social hostility toward out-group members (χ2 = 11.291, df = 1, p < 0.01). These findings suggest that even temporary group identities, formed in a sports setting, can lead to intergroup bias in adolescents’ social decision-making, promoting goodwill toward the in-group while restricting the autonomy of out-group members.