Millions of people suffer from acquired brain injury (ABI) each year, increasing the pressure on healthcare facilities. Personality functioning is thought to influence key factors for treatment success, such as motivation, coping strategies, and treatment adherence. In the ABI population no instruments are currently validated to assess personality functioning. This study therefore analysed the internal consistency and test-retest reliability of two questionnaires in the ABI population, namely the LPFS-BF 2.0 and the SIPP-SF. Next, the LPFS-BF 2.0 was examined to indicate its suitability as a brief instrument to assess personality functioning in this population. Lastly, the profile of personality functioning as measured with the SIPP-SF was explored in psychiatric and non-psychiatric patients with ABI. In this study, both questionnaires showed good psychometric qualities in the ABI population. The shorter LPFS-BF 2.0 demonstrated to be a suitable brief instrument for the SIPP-SF. In general, psychiatric patients show more maladaptive personality functioning than non-psychiatric patients with ABI, but the overall level of functioning in both groups is low compared to previous studies. Further research is needed to examine if maladaptive personality functioning does indeed lead to a longer and less satisfactory rehabilitation trajectory.