Depression in adolescence is a widespread problem and leads to extensive psychosocial impairments. For these reason, a resource diary has been developed as a four-week positive writing intervention. The aims of this intervention were to enhance well-being, to promote emotion regulation and resource realization, as well as to prevent symptoms of depression in adolescence. Seventy-seven students of 8th form were randomized either to a positive writing group (n = 38) or to a neutral writing group (n = 39). At the post-test, the positive writing group showed a lower dysphoric mood, fewer worries and less rumination in comparison with the neutral writing group. No effects on resource realization were detected. This study provides the first evidence of the beneficial effects of positive writing on indicators of well-being and depressive symptoms. In conclusion, the resource diary represents an economical intervention for preventing depression in adolescence.