BACKGROUND: Medication non-adherence is a leading cause of treatment failure in psychiatric populations. However, current studies highlight the lack of methodological guidance on medication assessments. Ecological Momentary Assessment (EMA), using smartphone-based evaluations, shows promise for real-time monitoring in everyday settings. AIMS: This study evaluated EMA's effectiveness in assessing medication adherence in patients with schizophrenia, depression, and substance use disorders (SUD), covering various treatment regimens. MATERIALS & METHODS: A total of 133 participants (27 with schizophrenia, 20 with depression, 44 with SUDs, and 42 healthy controls) completed EMA via study-provided smartphones five times daily over 1 week. Treatment regimens, categorized by mono vs. polytherapy and single vs. multiple daily doses, were documented. EMA adherence was calculated from the completion rate of the assessments, while medication adherence was assessed daily for patients. Both mean medication adherence and adherence variation over time were analysed by diagnosis and treatment regimen. RESULTS: All groups demonstrated high mean EMA and medication adherence, with minor variations across treatment types. Importantly, patients showed improved adherence over time, independently of diagnosis or regimen. DISCUSSION: These findings indicate EMA's potential as an effective method for capturing medication adherence in psychiatric populations. CONCLUSION: The approach's capacity for real-time, context-sensitive data collection could reveal adherence patterns and changes not detectable by conventional methods, offering valuable insights for clinical practice.