The main behavioral characteristic of subthreshold depression that is observed in adolescents is the low frequency of exposure to environmental rewards. Therefore, it was considered that a simple intervention conducted in short sessions, focusing on increasing access to positively reinforcing activities, would be efficacious in increasing the availability of rewards. We conduct a randomized controlled trial to examine the efficacy of such a behavioral activation program that was conducted weekly for 5 weeks in 60-min sessions. Late adolescent university students aged 18–19 years with subthreshold depression were randomly allocated to a treatment (n = 62) or a control group (n = 56). The primary outcome of the study was the Beck Depression Inventory-II score. Results indicated that late adolescent students in the treatment group showed significant improvements in their depressive symptoms (effect size −0.90, 95 % CI −1.28 to −0.51) compared to the control group. Students in the treatment group also showed significant improvements in self-reported rating of quality of life and in behavioral characteristics. It is concluded that this intervention had a large and significant effect despite being short and simple and that this low-intensity cognitive behavioral therapy program could be conducted in many different types of institutions. It is suggested that the long-term effects of the treatment program should be targeted for investigation in future studies.