Amongst the impacts of growing up with a parent with mental health challenges is the experience of stigma-by-association, in which children and young people experience impacts of stigmatisation due to their parent’s devalued identity. This article seeks to expand our understanding of this issue through an abductive analysis of qualitative data collected through a codesign process with young people. Results indicate that young people’s experiences of stigmatisation can be effectively understood as experiences of epistemic injustice. Participants expressed that their experiences comprised ‘more than’ stigma, and their responses suggest the centrality to their experiences of being diminished and dismissed in respect of their capacity to provide accurate accounts of their experiences of marginalisation and distress. Importantly, this diminishment stems not only from their status as children, and as children of parents with mental health challenges but operates through a range of stigmatised identities and devalued statuses, including their own mental health status, sexual minoritisation, disability and social class. Forms of epistemic injustice thus play out across the social and institutional settings they engage with. The psychological and social impacts of this injustice are explored, and the implications for our understanding of stigma around family mental health discussed.