Background: The nationwide Danish Cancer Registry and the Danish Melanoma Database both record data on melanoma for purposes of monitoring, quality assurance, and research. However, the data quality of the Cancer Registry and the Melanoma Database has not been formally evaluated. Methods: We estimated the positive predictive value (PPV) of melanoma diagnosis for random samples of 200 patients from the Cancer Registry (n = 200) and the Melanoma Database (n = 200) during 2004-2014, using the Danish Pathology Registry as "gold standard" reference. We further validated tumor characteristics in the Cancer Registry and the Melanoma Database. Additionally, we estimated the PPV of in situ melanoma diagnoses in the Melanoma Database, and the sensitivity of melanoma diagnoses in 2004-2014. Results: The PPVs of melanoma in the Cancer Registry and the Melanoma Database were 97% (95% CI = 94, 99) and 100%. The sensitivity was 90% in the Cancer Registry and 77% in the Melanoma Database. The PPV of in situ melanomas in the Melanoma Database was 97% and the sensitivity was 56%. In the Melanoma Database, we observed PPVs of ulceration of 75% and Breslow thickness of 96%. The PPV of histologic subtypes varied between 87% and 100% in the Cancer Registry and 93% and 100% in the Melanoma Database. The PPVs for anatomical localization were 83%-95% in the Cancer Registry and 93%-100% in the Melanoma Database. Conclusions: The data quality in both the Cancer Registry and the Melanoma Database is high, supporting their use in epidemiologic studies.