AbstractRobust reward sensitivity may help preserve mental well‐being in the face of adversity and has been proposed as a key stress resilience factor. Here, we present a mobile health application, “Imager,” which targets reward sensitivity by training individuals to create mental images of future rewarding experiences. We conducted a two‐arm randomized controlled trial with 95 participants screened for reward sensitivity. Participants in the intervention group received an ecological momentary intervention—Imager, which encouraged participants to create mental images of rewarding events for 1 week. The control group participants received only ecological momentary assessment, without the instruction to generate mental images. Adherence to Imager was high; participants in the intervention group engaged in 88% of the planned activities. In the follow‐up assessment, the intervention group reported less mental health symptoms, mainly in depression (β = −0.34, df = 93, p = .004) and less perceived stress (β = −0.18, df = 93, p = .035), than control group participants and compared with the baseline assessment. Our results show the positive effects of Imager on mental health symptoms. The encouraging effects of the app on mental health outcomes may lead to greater use of ecological momentary interventions in the clinical preventive practice of affective disorders.