Background: Workplace bullying may severely affect mental health, but its association with suicidal behaviour is unknown. We analysed whether individuals reporting exposure to workplace bullying had higher risk of suicidal behaviour than those not reporting such exposure. Methods: Using a prospective cohort study design, we linked data from nine Danish questionnaire-based surveys (2004-2014) to national register data until December 31st, 2016. Exposure to workplace bullying was measured by a single item. Suicide attempts were identified in hospital registers and death by suicide in the Cause of Death Register. Among participants with no previous suicide attempts, we estimated Hazard Ratios (HR) and 95% Confidence Intervals (CI), adjusting for sex, age, marital status, socio-economic status, and history of psychiatric morbidity. Outcomes: The sample consisted of 98 330 participants (713 809 person-years); 63·6% women, mean age 44·5 years. Workplace bullying was reported by 10 259 participants (10·4%). During a mean follow-up of 7·3 years, we observed 186 cases of suicidal behaviour, including 145 suicide attempts, 36 deaths by suicide and 5 cases that died by suicide after surviving a suicide attempt. The fully-adjusted HR for the association between workplace bullying and suicidal behaviour was 1·70 (95% CI 1·10–2·61). The HRs for suicide attempts and death by suicide were 1·72 (1·14–2·58) and 2·29 (1·00–5·53), respectively. A multiplicative interaction with sex (2·78, CI 1·59–4·88) indicated that workplace bullying was associated with suicidal behaviour among men. Interpretation: The results suggested an elevated risk of suicidal behaviour among men reporting workplace bullying. Funding Information: Region of Southern Denmark (grant number A1763); Danish Working Environment Research Fund (grant number 20130023294); Danish Working Environment Research Fund (grant number 10-2019-03). Declaration of Interests: The authors declare no competing interests. Ethics Approval Statement: The project has been approved by the Danish Data Protection Agency (Capital Region of Denmark; j.nr.: 2012-58-0004).