The Best Possible Self intervention (BPS) has demonstrated efficacy in promoting well-being in various populations, yet its impact in adolescence is under-researched. Our study investigated the feasibility and efficacy of the BPS in early adolescence (11–15 years) to promote positive affect and reduce negative affect. We conducted a randomized controlled online trial (N = 200, Mage = 14.01 years, SD = 1.19, 78.5 % female). Participants were assigned to the BPS group (n = 59), a writing control group (n = 68), or a non-writing control group (n = 73). Affect (PANAS-C-SF) was measured immediately before and after the intervention. The BPS demonstrated feasibility in our sample and significantly increased positive affect post-intervention compared to both control groups, suggesting a mood-boosting effect. The BPS did not significantly reduce negative affect post-intervention relative to the control groups. Our findings provide initial evidence that the BPS is a feasible and effective intervention for enhancing positive affect in early adolescence. Future research should explore its long-term effects, repeated administration, and potential for implementation in group settings to maximize its impact.