The high, and still rising, rate of loneliness is a threat to public health (Office of the Surgeon General, 2023), with negative mental and physical health consequences (e.g., Holt-Lunstad, 2021). Given that loneliness is a risk factor for poor mental health, efforts to address loneliness are urgently needed. Workplaces can facilitate an employee's social connection through supervisor support training, which can help mitigate loneliness. Among occupational groups, the military is at higher risk for mental health disorders, suicide, and loneliness (Fikretoglu et al., 2022; Naifeh et al., 2019). This study evaluated the efficacy of an evidence-based supportive-leadership training intervention targeting active-duty U.S. Army platoon leaders and targeting both proactive support behaviors that help bolster employee social connection and responsive support behaviors, including destigmatizing mental health. Ninety-nine platoon leaders (69.7% of eligible leaders) completed the 90-min training that consisted of both in-person and computer-based components. Using a cluster-randomized controlled trial design, intervention effects were tested using an intent-to-treat approach and revealed a significant effect, whereby loneliness of service members whose leaders were randomized to the intervention group (N = 118) was significantly reduced compared to loneliness reports for service members in the control group (N = 158). Additionally, service members with higher baseline loneliness were more strongly and positively impacted by the supervisor training, reporting higher levels of supportive behaviors from their leaders at 3 months postbaseline. In sum, these results suggest how workplaces, especially those that are considered high-risk occupations, and their leaders play a critical role in a national strategy to address Americans' well-being. (PsycInfo Database Record (c) 2024 APA, all rights reserved).