Objective Recent evidence supports that gynaecomastia may predict long-term morbidity, but evidence on the association with death and causes of death in males with gynaecomastia is lacking. The objective of this work is to estimate the risk of death in men diagnosed with gynaecomastia and evaluate whether this was conditional on underlying aetiologies of gynaecomastia. Design A nationwide register-based cohort study. Setting Nationwide Danish national health registries. Participants Males were diagnosed with incident gynaecomastia (n=23 429) from 1 January 1995 to 30 June 2021, and each was age and calendar matched to five randomly population-based males without gynaecomastia (n=117 145). Interventions Not applicable. Primary and secondary outcomes Gynaecomastia was distinguished between males without (idiopathic) and males with a known pre-existing risk factor. Cox regression models and Kaplan-Meier analyses estimated associations between gynaecomastia and death (all cause/cause specific). Results We identified a total of 16 253 males with idiopathic gynaecomastia and 7176 with gynaecomastia and a known pre-existing risk factor. Of these, 1093 (6.7%) and 1501 (20.9%) died during follow-up, respectively. We detected a 37% increased risk of all-cause death in males with gynaecomastia in the entire cohort (HR 1.37; 95% CI 1.31 to 1.43). Death risk was highest in males diagnosed with gynaecomastia and a known pre-existing risk factor (HR 1.75; 95% CI 1.64 to 1.86) compared with males with idiopathic gynaecomastia (HR 1.05; 95% CI 0.98 to 1.13). Specific causes of increased death were malignant neoplasms and circulatory, pulmonary and gastrointestinal diseases. Of the latter, an over fivefold risk of death from liver disease was detected (HR 5.05; 95% CI 3.97 to 6.42). Conclusions Males diagnosed with gynaecomastia are at higher risk of death, observed mainly in males with a known pre-existing risk factor of gynaecomastia. These findings will hopefully stimulate more awareness among healthcare providers to potentially apply interventions that aid in alleviating underlying risk factors in males with this condition.