Abstract Background The Externalizing Spectrum Inventory-Brief Form (ESI-BF) [1] is a 160-item self-report instrument designed for the assessment of externalizing psychopathology, yet few studies to date have evaluated its psychometric properties, structural fit, and criterion validity in forensic psychiatric settings. Methods Here, we investigated these aspects in a sample of forensic psychiatric inpatients (n = 77) from a maximum-security forensic psychiatric hospital in Sweden. We firstly investigated the reliability. Secondly, using confirmatory factor analysis, the structure of the ESI-BF. And thirdly, using a Bayesian approach, assessed how the three ESI-BF subfactors relate to criterion measures of antisocial behaviors, substance use, and lifetime externalizing spectrum diagnoses. Results The ESI-BF demonstrated good to adequate reliability and internal consistency, with all but four facet scales exhibiting α and ω values ≥ 0.80. Average inter-item correlations for the facet scales ranged from 0.31 to 0.74. However, all structural models exhibited poor to mediocre fit, with model fit values for the CFI being 0.66, 0.79 and 0.87 and RMSEA values of 0.14, 0.12 and 0.09. for the unidimensional correlated factors and bifactor model, respectively. Regarding criterion validity, all subscales of the item-based ESI-BF three-factor model exhibited robust correlations with the Life History of Aggression total, aggression and antisocial/consequences subscales, with correlations ranging from 0.29 to 0.55. All ESI-BF subfactors demonstrated robust associations, yet with different externalizing outcomes, lending tentative support to its criterion validity. Conclusion Despite remaining ambiguities regarding its structural fit, the ESI-BF may be promising for assessing externalizing psychopathology in forensic psychiatric populations. However, further investigation of the ESI-BF is needed before any firm conclusions can be drawn about its appropriateness in forensic psychiatric settings.