Stress First Aid is an evidence-informed peer-to-peer support intervention to mitigate the effect of the COVID-19 pandemic on the well-being of health care workers (HCWs). To evaluate the effectiveness of a tailored peer-to-peer support intervention compared with usual care to support HCWs' well-being at hospitals and federally qualified health centers (FQHCs) during the COVID-19 pandemic. This cluster randomized clinical trial comprised 3 cohorts of HCWs who were enrolled from March 2021 through July 2022 at 28 hospitals and FQHCs in the US. Participating sites were matched as pairs by type, size, and COVID-19 burden and then randomized to the intervention arm or usual care arm (any programs already in place to support HCW well-being). The HCWs were surveyed before and after peer-to-peer support intervention implementation. Intention-to-treat (ITT) analysis was used to evaluate the intervention's effect on outcomes, including general psychological distress and posttraumatic stress disorder (PTSD). The peer-to-peer support intervention was delivered to HCWs by site champions who received training and subsequently trained the HCWs at their site. Recipients of the intervention were taught to respond to their own and their peers' stress reactions. Primary outcomes were general psychological distress and PTSD. General psychological distress was measured with the Kessler 6 instrument, and PTSD was measured with the PTSD Checklist. A total of 28 hospitals and FQHCs with 2077 HCWs participated. Both preintervention and postintervention surveys were completed by 2077 HCWs, for an overall response rate of 28% (41% at FQHCs and 26% at hospitals). A total of 862 individuals (696 females [80.7%]) were from sites that were randomly assigned to the intervention arm; the baseline mean (SD) psychological distress score was 5.86 (5.70) and the baseline mean (SD) PTSD score was 16.11 (16.07). A total of 1215 individuals (947 females [78.2%]) were from sites assigned to the usual care arm; the baseline mean (SD) psychological distress score was 5.98 (5.62) and the baseline mean (SD) PTSD score was 16.40 (16.43). Adherence to the intervention was 70% for FQHCs and 32% for hospitals. The ITT analyses revealed no overall treatment effect for psychological distress score (0.238 [95% CI, -0.310 to 0.785] points) or PTSD symptom score (0.189 [95% CI, -1.068 to 1.446] points). Post hoc analyses examined the heterogeneity of treatment effect by age group with consistent age effects observed across primary outcomes (psychological distress and PTSD). Among HCWs in FQHCs, there were significant and clinically meaningful treatment effects for HCWs 30 years or younger: a more than 4-point reduction for psychological distress (-4.552 [95% CI, -8.067 to -1.037]) and a nearly 7-point reduction for PTSD symptom scores (-6.771 [95% CI, -13.224 to -0.318]). This trial found that this peer-to-peer support intervention did not improve well-being outcomes for HCWs overall but had a protective effect against general psychological distress and PTSD in HCWs aged 30 years or younger in FQHCs, which had higher intervention adherence. Incorporating this peer-to-peer support intervention into medical training, with ongoing support over time, may yield beneficial results in both standard care and during public health crises. ClinicalTrials.gov Identifier: NCT04723576.