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Background: Internet psychoeducational interventions improve employees' mental health. However, implementing them for employees in micro- and small-sized enterprises (MSEs) is challenging. Objectives: This randomized controlled trial examined the effectiveness of a fully automated text-based stress management program, “WellBe-LINE,” in improving mental health and job-related outcomes for employees in workplaces with fewer than 50 employees. Methods: The program was developed based on stakeholder interviews and surveys of 1000 employees at MSEs. Adult full-time employees at an enterprise with fewer than 50 employees were recruited from registered members of a web survey company in Japan. Participants were randomly allocated to the intervention or control group (1:1). Participants in the intervention group were invited to register for the program using the LINE app. Psychological distress measured by Kessler 6 (K6) was a primary outcome, with self-administrated questionnaires at baseline, 2-month (post), and 6-month follow-ups. A mixed model for repeated measures conditional growth model analysis was conducted using a group ∗ time interaction as an intervention effect. Implementation outcomes were measured through implementation outcome scales for digital mental health (iOSDMH). Results: 1021 employees were included in this study. No significant effects were shown in any outcome. The reported implementation outcomes were positively evaluated, with 80 % acceptability, 86 % appropriateness, and feasibility (ease of understanding the contents [88 %], frequency [86 %], and length of content [86 %]). Conclusions: A simple text-message program for employees at MESs was acceptable, appropriate, and feasible; however, it did not result in improved mental health or job-related outcomes. Trial registration: UMIN clinical trial registration: UMIN000050624 (registration date: March 18, 2023).