This study tests the efficacy of a unique resilience-strengthening intervention using a clustered-randomized controlled trial. It was hypothesized that the training, which encourages adaptive self-reflection on stressor events and the effectiveness of coping strategies and resources, would exert a positive effect on mental health outcomes via increased reflection and decreased brooding. The trial was conducted during a significant stressor period with a final sample of 204 second-class Officer Cadets from the Royal Military College, Australia. Platoons of Cadets were randomly allocated to either Self-Reflection Resilience Training (SRT; n = 96) or an exposure-matched active control group that received training as usual (i.e., cognitive-behavioral skill development training) and communication skills seminars (n = 108). Compared to the active control group, SRT was more effective at preventing the onset of depression symptoms and promoting stable levels of perceived stress during a period of increased exposure to training stressors, consistent with a resilient trajectory. The Self-Reflection group unexpectedly demonstrated higher anxiety symptoms than the Control group at immediate follow-up, but these symptoms returned to baseline levels at longer term follow-up. In contrast, the Control group experienced increasing anxiety symptoms between immediate and longer term follow-up. Mediation analyses supported an indirect effect of SRT on all three outcome measures via brooding, but not via reflection. This study provides support for the capacity of a practical, sustainable, and scalable intervention based on self-reflection to strengthen resilience in the military training setting. (PsycInfo Database Record (c) 2021 APA, all rights reserved).