BackgroundText-based self-guided internet-based stress management programs can improve mental health among workers. However, such mental health programs have scarcely been implemented in micro– and small-sized enterprises (MSEs), which are lacking in occupational healthcare services.ObjectivesThis paper describes a study protocol for a pre-post feasibility study of an effectiveness-implementation hybrid type 2 trial of text-based internet-based programs (“WellBe-LINE”) in MSEs with less than 50 employees. This feasibility study primarily aims to evaluate trial methods for future effectiveness-implementation hybrid type 2 trials.MethodsFor this study protocol, an internet– and text-based self-care intervention program using the LINE app (a popular message tool in Japan) will be prepared according to evidence-based psychoeducational topics. Based on our online survey findings, personalized algorithms will be implemented according to employees’ gender, age, and psychological distress levels. A pre-post design feasibility study will be conducted on ten companies to evaluate trial methods (e.g., recruitment, penetration, and procedures). The primary outcome will involve individual-level penetration, defined as the proportion of the number of employees who register for the program divided by the total number of invited employees at the company. The progression criterion specifies that more than 50% of the recruited companies obtain 60% individual penetration. Finally, acceptability, appropriateness, feasibility, and cost will be measured using quantitative internet-based questionnaires and qualitative interviews.DiscussionThis pre-post feasibility study for future effectiveness-implementation hybrid type 2 trials will provide in-depth knowledge about the successful implementation of text-based, semi-personalized, self-care mental health interventions in real-world settings using both quantitative and qualitative data.ConclusionsThis feasibility study will help validate the effectiveness of text-based interventions using a widely used social networking service (SNS) tool for employees in MSEs.Trial registrationUMIN clinical trial registration: UMIN000046960 (registration date: February 21, 2022)https://center6.umin.ac.jp/cgi-open-bin/ctr/ctr_view.cgi?recptno=R000053570Contributions to the literatureInternet-based mental health interventions in micro– and small-sized enterprises have not been implemented yet.This feasibility study plans to test the implementation strategies to achieve high penetration in employees.This study can provide insights into occupational health implementation in a disadvantaged context.