Ethical guidelines state that psychologists should consider clients' religion in their practice. However, some clients have reported negative experiences regarding clinicians' treatment of religion in psychotherapy. These experiences may constitute microaggressions, which have been negatively associated with the working alliance and treatment outcomes among clients with various identities (e.g., those of marginalized racial groups or sexual orientations). The present study used mixed methods to examine religious microaggressions among current and former psychotherapy clients identifying as religious (N = 396). Approximately 39% of participants indicated that at least one religious microaggression occurred during treatment; the most common was minimization or avoidance of religious issues. Religious microaggressions were negatively associated with the working alliance and outcomes. Additionally, the working alliance fully mediated the association between religious microaggressions and poorer outcomes. Thematic analysis of qualitative descriptions of religious microaggressions yielded seven themes: minimization/avoidance of religious issues, assumptions of religious homogeneity, pathologizing religion, unhelpful/inappropriate interventions, pressure to embrace religion, prioritization of therapist's religious beliefs, and lack of expertise. Limitations include a retrospective, cross-sectional design and a majority White, female, and Christian sample. These results provide initial evidence that a substantial minority of religious clients may experience religious microaggressions in psychotherapy, which could impede treatment progress via negative effects on the working alliance. As such, clinicians may be able to enhance client outcomes by increasing awareness of religious microaggressions in their work and addressing microaggression-related alliance ruptures openly when they do occur. Implications for training are also discussed. (PsycInfo Database Record (c) 2022 APA, all rights reserved).