<p>The Personality Inventory for DSM-5-Brief Form (PID-5-BF) has become widely used for providing efficient self-report assessment of trait domains from the Alternative Model for Personality Disorders (AMPD). Existing studies of the PID-5-BF’s psychometric properties have been informative, but they have often relied on monomethod assessment. We advance research on the PID-5-BF’s psychometrics and understanding of multimethod personality-psychopathology associations by conducting analyses integrating self-report, informant, and interview measures. As a second related aim, we examined the degree of self-informant convergence for AMPD ratings. Adult participants (<i>N</i> = 235; 44% currently accessing psychotherapy) completed self and interview measures of psychopathology and substance use, with a subset of these participants (<i>n</i> = 116) also contributing informant data. Results indicated that PID-5-BF scores generally showed theoretically consistent mono- and cross-method associations, though some specific findings indicated possible discriminant validity issues for select PID-5-BF scales. Self and informant AMPD trait ratings also converged moderately, with self-rated negative affectivity and psychoticism mean levels slightly exceeding informant-rated mean levels. Thus, our study presents multimethod results supporting the self-report PID-5-BF’s validity overall, while also advancing understanding of multimethod trait associations when using brief trait measures such as the PID-5-BF. We draw on these results to discuss ideas for sharpening dimensional assessment in research on personality disorders specifically and transdiagnostic psychopathology more generally.</p>