Autistic people experience a higher prevalence of self-harming behaviours than do the general population. Self-harm remains a stigmatised topic, and until recently, self-harming behaviours in autism were considered to be limited to self-injurious behaviours experienced by intellectually impaired autistic children and not to be experienced by cognitively able autistic adults. Because of the belief held by many professionals that many mental health-related problems are inherently part of autism and immune to treatment or help, many autistic people are left unable to access the help and support that they desperately need. This study adopts an online qualitative methodology to explore this under-researched phenomenon, thematically analysing online forum posts from autistic adults to determine what forms of self-harm are described, what precipitates the self-harm and how forum users support each other. The findings reveal that self-harming behaviours are nuanced and complex, highlighting the connections between sensory overload, meltdowns, stimming and impulsive repetitive blunt trauma seen as autistic self-injurious behaviours. When seeking help, barriers include navigating misdiagnoses of other mental illnesses and being misunderstood or disbelieved by professionals. Online forum users offer help in the form of emotional empathy, as well as practical solutions, providing an example of valuable peer support.