The aim of this study was to assess the extent to which discrepancy between self-reported and clinician-rated severity of depression are due to inconsistent self-reports. Response inconsistency threatens the validity of the test score. We used data from a large sample of outpatients (N = 5,959) who completed the self-report Beck Depression Inventory–II (BDI-II) and the clinician-rated Montgomery–Åsberg Depression Rating Scale (MADRS). We used item response theory based person-fit analysis to quantify the inconsistency of the self-report item scores. Inconsistency was weakly positively related to patient–clinician discrepancy (i.e., higher BDI-II scores relative to MADRS scores). The mediating effect of response inconsistency in the relationship between discrepancy and demographic (e.g., ethnic origin) and clinical variables (e.g., cognitive problems) was negligible. The small direct and mediating effects of response inconsistency suggest that inaccurate patient self-reports are not a major cause of patient–clinician discrepancy in outpatient samples. Future research should investigate the role of clinician biases in explaining clinician–patient discrepancy.