Background: Previous studies of pubertal timing and self-harm are limited by subjective measures of pubertal timing or by the conflation of self-harm with suicide attempts and ideation. The current study investigates the association between an objective measure of pubertal timing in females – age at menarche – and self-harm with and without suicidal intent in adolescence and adulthood. Method: Birth cohort study based on 4,042 females from the Avon Longitudinal Study of Parents and Children (ALSPAC). Age at menarche was assessed prospectively between ages 8 and 17 years. Lifetime history of self-harm was self-reported at ages 16 and 21 years. Associations between age at menarche and self-harm, both with and without suicidal intent, were examined using multivariable logistic regression. Results: Later age at menarche was associated with a lower risk of lifetime self-harm at age 16 years (OR per year increase in age at menarche 0.87; 95% CI 0.80, 0.95). Compared with normative timing, early menarche (<11.5 years) was associated with an increased risk of self-harm (OR 1.31; 95% CI 1.04, 1.64) and later menarche (> 13.8 years) with a reduced risk (OR 0.74; 95% CI 0.58, 0.93). The pattern of association was similar at age 21 years (OR per year increase in age at menarche 0.92, 95% CI 0.85, 1.00). There was no strong evidence for a difference in associations with suicidal versus non-suicidal self-harm. Conclusions: Risk of self-harm is higher in females with early menarche onset. Future research is needed to establish whether this association is causal and to identify potential mechanisms.