IMPORTANCE: Stress among health care professionals is well documented. The use of mindfulness-based interventions to reduce stress has shown promising results; however, the time commitment of typical programs can be a barrier to successful implementation in health care settings. OBJECTIVE: To determine the efficacy and feasibility of a brief mindfulness-based program to reduce stress during work hours among health care professionals. DESIGN, SETTING, AND PARTICIPANTS: This intent-to-treat randomized clinical trial was conducted among full-time health care professionals at the Clinical Center at the National Institutes of Health in Bethesda, Maryland, between September 2017 and May 2018. Participants were randomized to receive mindfulness-based self-care (MBSC) training or life-as-usual control. Data were analyzed from June 2018 to January 2020. INTERVENTIONS: The MBSC intervention included 5 weekly, 1.5-hour in-class mindfulness practice sessions. MAIN OUTCOMES AND MEASURES: Stress level was the primary outcome, assessed with the Perceived Stress Scale 10-Item version. Secondary outcomes included anxiety, burnout, positive and negative affect, mindfulness (trait and state), and self-care. Assessments were taken at baseline and at the end of the intervention (week 5) in the intervention and control groups, and at follow-up (week 13) in the intervention group to test for a maintenance effect. A postprogram evaluation was also obtained. RESULTS: Of 82 randomized participants, 78 who completed the study at week 5 were included in the modified intent-to-treat analysis (median [interquartile range] age, 32 [23-48] years; 65 [83%] women), including 43 participants in the MBSC group and 35 participants in the control group. At the end of the intervention, compared with the control group, the MBSC group had reduced levels of stress (mean [SD] score, 17.29 [5.84] vs 18.54 [6.30]; P = .02) and anxiety (mean [SD] score, 2.58 [1.52] vs 4.23 [1.73]; P < .001), and improved positive affect (mean [SD] score, 35.69 [7.12] vs 31.42 [7.27]; P < .001), state mindfulness (mean [SD] score, 3.74 [1.18] vs 2.78 [1.16]; P < .001), and mindful self-care (mean [SD] score, 7.29 [2.44] vs 5.54 [2.77]; P < .001). Burnout, negative affect, and trait mindfulness levels did not differ between groups. Changes within the MBSC group through follow-up included sustained reductions in stress (change, -6.14; 95% CI, -7.84 to -4.44; P < .001), anxiety (change, -1.46; 95% CI, -1.97 to -0.94; P < .001), trait mindfulness (change, 0.63; 95% CI, 0.36 to 0.90; P < .001), and state mindfulness (change, 1.89; 95% CI, 1.39 to 2.39; P < .001). CONCLUSIONS AND RELEVANCE: This randomized clinical trial found that this brief mindfulness-based intervention was an effective and feasible means to reduce stress in health care professionals. Larger studies are needed to assess the effects on clinical care and patient outcomes. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT03781336.