Summary Background  Giant congenital melanocytic naevi (GCMN) are known risk factors for the development of melanoma. However, melanoma risk among Asians is rarely evaluated. Objectives  To evaluate the clinical characteristics and risk of melanoma development from GCMN in Koreans, we performed a nationwide retrospective cohort study in Korea. GCMN were defined as those comprising ≥ 5% body surface area in children or measuring ≥ 20 cm in adults. Methods  In total, 131 patients with GCMN were enrolled, with a mean age of 10·3 years (range: birth–70 years). Results  The posterior trunk was the most common site (67, 51·1%), followed by lateral trunk, anterior trunk, legs, both anterior and posterior trunk, buttocks, and arms. Satellite naevi were present in 69 cases (52·7%), and axial areas were more commonly involved in patients with satellite naevi than in those without satellite lesions. Atypical features such as rete ridge elongation and bridges were seen, and, among these, pagetoid spread and ballooning cell changes were more common in patients < 4 years old. Proliferative nodules were found in three cases. Melanomas had developed in three of 131 patients (2·3%; a 6‐year‐old girl, a 14‐year‐old girl and a 70‐year‐old man), and the incidence rate was 990 per 100 000 person‐years. Melanomas in these three patients consisted of two cutaneous melanomas and one extracutaneous meningeal melanoma. Conclusions  We should be aware of melanoma development from GCMN, and lifelong follow‐up is required due to the risk of melanoma arising in GCMN.