Purpose <p>This study aimed to (i) identify how carers and healthcare professionals define and describe brain tumour–related personality and behaviour changes in adults and (ii) explore the impact of these changes on family carers.</p> Methods <p>We conducted 22 semi-structured interviews with neuro-oncology healthcare professionals working in Australia and 13 interviews with current or bereaved carers in Australia and Denmark. Definition data were analysed deductively using the Diagnostic and Statistics Manual of Mental Disorders 5th Edition Text Revision (DSM-5-TR) Personality Change Due to Another Medical Condition diagnostic criteria, and inductively to uncover new ideas related to descriptions of brain tumour–related personality and behaviour changes perceived by carers and healthcare professionals. Inductive thematic analysis was conducted to develop themes exploring the impact of patient personality and behaviour changes on carers.</p> Results <p>Two themes were identified from participants’ description of personality and behaviour changes associated with brain tumours: (i) variable presentation and multifactorial aetiology and (ii) manageable versus challenging changes. The specific features and symptoms of personality and behaviour changes reported by the sample are presented alongside the DSM-5-TR features of Personality Change Due to Another Medical Condition for clinical utility.</p> <p>Three themes were identified related to the impact of patient brain tumour–related personality and behaviour changes on carers: (i) grieving the person: “I don’t know who this person is anymore”; (ii) bearing the brunt of behavioural changes: “Happening behind closed doors”; and (iii) safety concerns and aggression: “I didn’t know what was going to happen next”. The three main themes were further elaborated through corresponding subthemes.</p> Conclusions <p>This study highlights the nuances and complexity in conceptualising personality changes in patients with a brain tumour and the grief, isolation, and safety concerns experienced by carers. Brain tumour–related aggression was identified as a significant concern by both healthcare professionals and carers, lacking clinical guidelines internationally for managing violence and aggression in this population. Future research is required to test interventions and support for safeguarding and risk management for patients and their family members.</p>