Introduction: Psychological distress and fatigue are common in persons with diabetes, adversely affecting quality of life and complicating diabetes self-management. Offering diabetes-specific self-guided cognitive behavioral therapy (CBT) may be helpful for persons with diabetes and mild symptoms of psychological distress and fatigue. We are the first to test the feasibility and user experiences of a web-based self-help app called ‘MyDiaMate’ in adults with type 1 and type 2 diabetes. Methods and materials: MyDiaMate was developed in close collaboration with persons with diabetes and professionals, building on elements from existing (guided) diabetes-specific CBT interventions. The study was advertised, offering free access to the app for adults with diabetes for a period of three months. Feasibility and user experiences were tested in a non-randomized study with pre- and post- measurements and interviews in a small sample. In addition usage of the app was studied using log-data. Results: In total N = 55 adults with diabetes signed up for the study. Mean age was M = 42.7 (SD = 15.6), mostly women (n = 39, 70.9%), higher educated (n = 36, 65.5%), and diagnosed with type 1 diabetes (n = 37, 67.3%). About half reported current or a history of psychological complaints. All the participants completed baseline assessments, and n = 32 participants (58%) completed the follow-up questionnaire. Main reasons for participating in the study were: to preserve or improve mental fitness (40.6%), curiosity (25.0%) and wanting to contribute to research (34.4%). No major technical issues were encountered in accessing or using the app. The app was opened at least once by n = 51 participants, median use of the modules was 28 min (1–80) within a period of 1 to 92 days (median = 10). Almost all participants (n = 50, 98.0%) opened the basic module ‘Diabetes in balance’, of whom 32 (62.7%) completed this module. ‘My mood’ and ‘My energy’ were opened by n = 40 (78.4%) and n = 32 (62.7%) participants, respectively, and completed by n = 21 (52.5%) and n = 9 (28.1%) of the participants. Of all participants, 40.6% would recommend the app to others living with diabetes. Conclusions: This study confirmed the feasibility of MyDiaMate as a diabetes-specific self-guided app for adults wishing to preserve or improve their psychological health. While user experiences were overall positive, further tailoring the content to individual needs and preferences could enhance uptake, usage and appreciation. Future research should explore its effectiveness in a randomized controlled trial.