BACKGROUND: Growing evidence suggests that self-harm-related mental imagery is involved in the transition from self-harm ideation to enactment. However, there has been little research on this important phenomenon in adolescent populations. METHODS: Using an online questionnaire, the frequency, content and impact of self-harm-related mental imagery was investigated in a transdiagnostic clinical population of young people with recent self-harm. Mood ratings were used to assess the impact of completing the questionnaire. RESULTS: Fifty-five young people aged 14-24 years old completed the study. Participants were mostly female (85.5%) and White (87.3%). All but one participant (98.2%) reported images related to self-harm, with 53 (96.4%) reporting future-oriented self-harm images, 53 (96.4%) reporting past-oriented images, and 52 (94.5%) reporting both. Imagery included imagining self-harm and particularly dangerous acts (including suicide), specific methods, and the consequences of self-harm for self and others. Past self-harm-related mental imagery was sometimes used to develop future-oriented self-harm-related imagery planning, highlighting the influence of previous exposure to self-harm. Most participants (N = 45; 88.2%) stated that significant self-harm-related mental imagery increased the likelihood they would self-harm. Stimulation of mental imagery was most frequently reported to be related to dreams about self-harm (N = 33; 60.0%), and exposure to self-harm-related content on social media (N = 32; 58.2%) and in fictional TV programmes (N = 30; 54.6%). There was no significant difference in participants' pre- and post-questionnaire mood ratings. CONCLUSIONS: Self-harm-related mental imagery is commonly experienced by young people who self-harm and may play a role in ideation-to-enactment of self-harm. Asking about self-harm-related mental imagery can be done safely and could be considered for inclusion in routine clinical assessments. Self-harm exposure and the origins of self-harm-related imagery, such as the links with past self-harm and social media, as well as potential imagery-based interventions for self-harm, require further evaluation. A working model of self-harm-related mental imagery is presented.