Background: Structural and behavioral interventions to manage work-related stress are effective in employees. Nonetheless, they have been implemented insufficiently, particularly in micro- and small-sized enterprises (MSE). Main barriers include a lack of knowledge and limited resources, which could potentially be overcome with simplified web-based alternatives for occupational stress prevention. However, there is a lack of implementation research about web-based prevention in realistic settings of MSE. Objective: The aim of this study is to evaluate the implementation process and success of an integrated web-based platform for occupational stress prevention (“System P”) and to identify potential barriers for its uptake and use in MSE in Germany. Methods: This study with a mixed-methods approach investigates eight process-related outcomes in a quantitative part I (adoption, reach, penetration, fidelity/dose, costs, acceptability) and a qualitative part II (acceptability, appropriateness and feasibility). Part I has a pre-post design with two measurements (6 months apart) with 98 individual participants and part II consists of 12 semi-structured interviews with managers and intercorporate stakeholders. Results: Part I revealed shortcomings in the implementation process. Adoption/Reach: Despite extensive marketing efforts, less than 1% of the contacted MSE responded to the offer of System P. A total of 40 MSE registered, 24 of which, characterized by good psychosocial safety climate, adopted System P. Penetration: Within these 24 MSE, 15% of the employees used the system. Fidelity/Dose: 11 MSE started a psychosocial risk-assessment (PRA), and no MSE finished it. The stress-management training (SMT) was started by 25 users and completed by 8. Costs: The use of System P was free of charge, but the time required to engage with was an indirect cost. Part II added insights on the perception of the web-based intervention: Acceptance of System P by users and stakeholders was good and it was assessed as appropriate for MSE. Results for feasibility were mixed. Conclusions: Although System P was generally perceived as useful and appropriate, only a small number of contacted MSE implemented it as intended. Prior experience and sensitivity for occupational (stress) prevention were mentioned as key facilitators, while (perceived) indirect costs were a key barrier. Enabling MSE to independently manage stress prevention online did not result in successful implementation. Increasing external support could be a solution. ⁺ Full project name: “PragmatiKK – Pragmatische Lösungen für die Implementation von Maßnahmen zur Stressprävention in Kleinst- und Kleinbetrieben” (= Pragmatic solutions for the implementation of stress prevention interventions in micro and small-sized enterprises). Trial registration: German Register of Clinical Studies (DRKS) DRKS00026154, date of registration 2021-09-16.