Cancer-related fatigue (CRF) is one of the most common symptoms reported by people with primary brain tumour (BT). Previous research predominantly examined CRF using quantitative assessments, failing to capture the rich insight garnered from exploring individuals' lived experiences. We addressed this gap by qualitatively exploring people with BTs' experiences of CRF. Semi-structured interviews were conducted with people with BT, their caregivers, and healthcare professionals (HCPs) who care for them. Interviews explored the experience, impact, and management of CRF, including types of support provided by HCPs. Data were analysed using reflexive thematic analysis. Forty participants were interviewed (24 people with BT, 5 caregivers, 11 HCPs). Qualitative analysis identified four themes: pervasiveness of CRF; impacts of CRF; advice and support; and self-management strategies. CRF was described as an almost universal symptom with physical, emotional, and cognitive aspects and profound psychosocial and functional impacts. HCPs reported assessing fatigue and providing management support. Yet, people with BT and caregivers reported CRF assessment and support were rarely received. Consequently, people with BT developed their own management strategies. All participants identified a lack of CRF information resources and interventions specific to people with BT. Our findings provide rich insight into the pervasive, debilitating impact of CRF in people with BT and highlight the lack of BT-specific CRF support and information available. There is a critical need for evidence-based fatigue interventions and information resources tailored to the needs of people with BT.