Background: Few evidence-based mental health apps are widely available to patients and, conversely, many of the available apps have not been appropriately evaluated. Given that the ultimate goal is to scale-up and open internet-based platforms (IBP), it is crucial to appropriately evaluate their real-world feasibility beforehand. We aimed to evaluate the implementation feasibility of a smartphone-based psychoeducational programme for bipolar disorder, exploring its long-term retention, usability, perceived helpfulness and satisfaction, alongside its impact on secondary health outcomes. Methods: Participants were recruited via the project website after completing an online screening questionnaire. They were requested to complete web-based questionnaires before using the app and after 6 months of use which included sociodemographic, illness and treatment variables, the world health organisation-five well-being index (WHO-5) and the short form health survey (SF-36). The follow-up questionnaires also contained satisfaction and usefulness questions. Results: 201 participants took part in the study. According to their retention, 66.2% of the participants were classified as noncompleters and 33.8% as completers. The only predictor significantly associated with higher odds of retention was older age (OR = 1.021, p < 0.001). 62% of the users reported they were satisfied with the programme with a higher percentage among completers. Who-5 baseline and follow-up scores showed a significant improvement as well as 6 out of 8 domains of the SF-36. Limitations: Screening and outcome measures were administered using exclusively self-reported online methods. Conclusion: The 6-month attrition rate of the programme was high. Positive outcomes regarding satisfaction were found predominantly among completers. The optimal dosage and retention of IBP mental health programmes remain challenging issues that need further research.