The link between engagement in non-suicidal self-injury (NSSI)–related activities on SNS (e.g. viewing, commenting, sharing and uploading NSSI content) with body image and NSSI behaviour remains under researched in adolescents with EDs. The main aim of the current research was to examine associations between NSSI-related activities on SNS with body image and NSSI behaviour severity among female adolescents diagnosed with an ED. A total of 52 female adolescents (mean age = 15.35 years, SD = 1.49) diagnosed with an ED completed self-report questionnaires related to NSSI behaviour, SNS usage and body image. Participants were divided into two groups: low NSSI behaviour severity (from 0 to 10 NSSI behaviours; n = 28) and high NSSI behaviour severity (more than ten NSSI behaviours; n = 24). Within the high NSSI severity group, individuals that comment and share NSSI online content significantly reported higher negative body image. A hierarchical binary logistic regression showed that the frequency of NSSI online content on SNS emerged as significant predictor of NSSI behaviour severity within last year after controlling for body image and searching for ED content on SNS. Our findings suggest that not only searching for ED content, but also being daily engaged in NSSI online activities may increase the risk of NSSI behaviour severity in female adolescents with EDs.