BACKGROUND: Emotional exhaustion (EE) in health care workers is common and consequentially linked to lower quality of care. Effective interventions to address EE are urgently needed. OBJECTIVE: This randomized single-exposure trial examined the efficacy of a gratitude letter-writing intervention for improving health care workers' well-being. METHODS: A total of 1575 health care workers were randomly assigned to one of two gratitude letter-writing prompts (self- vs other focused) to assess differential efficacy. Assessments of EE, subjective happiness, work-life balance, and tool engagement were collected at baseline and 1-week post intervention. Participants received their EE score at baseline and quartile benchmarking scores. Paired-samples t tests, independent t tests, and correlations explored the efficacy of the intervention. Linguistic Inquiry and Word Count software assessed the linguistic content of the gratitude letters and associations with well-being. RESULTS: Participants in both conditions showed significant improvements in EE, happiness, and work-life balance between the intervention and 1-week follow-up (P<.001). The self-focused (vs other) instruction conditions did not differentially predict improvement in any of the measures (P=.91). Tool engagement was high, and participants reporting higher motivation to improve their EE had higher EE at baseline (P<.001) and were more likely to improve EE a week later (P=.03). Linguistic analyses revealed that participants high on EE at baseline used more negative emotion words in their letters (P=.005). Reduction in EE at the 1-week follow-up was predicted at the level of a trend by using fewer first-person (P=.06) and positive emotion words (P=.09). No baseline differences were found between those who completed the follow-up assessment and those who did not (Ps>.05). CONCLUSIONS: This single-exposure gratitude letter-writing intervention appears to be a promising low-cost, brief, and meaningful tool to improve the well-being of health care workers.