IMPORTANCE Determining the associated risk between papillary thyroid carcinoma (PTC) and cutaneous malignant melanoma (CM) and the rate of BRAF v600e mutation could help identify a common genetic component of these 2 cancers. OBJECTIVES To define the relative risk of PTC in patients with CM, and vice versa, and their first- through fifth-degree relatives and spouses by using a unique population research database; and to assess the rate of BRAF v600e mutation in a group of patients with both diagnoses. DESIGN, SETTING, AND PARTICIPANTS Retrospective rev iew using the Utah Population Database (which is linked to medical records and the Utah Cancer Registry from 1966 to 2011) and tissue analysis in a tertiary care facility. Included were 4460 patients diagnosed with PTC and 14 569 with CM in Utah between 1966 and 2011 and their first- through fifth-degree relatives and spouses. These were compared at a 5:1 ratio with matched, population-based controls. MAIN OUTCOMES AND MEASURES Statistically significant increased risk of PTC in patients with CM, and vice versa, and any first- through fifth-degree relatives and spouses; and a significantly higher rate of BRAF v600e mutation in patients with both PTC and CM than would be expected for each individual condition alone. RESULTS Patients with CM had a 2.3-fold increased risk (P < .001) of being diagnosed as having PTC compared with population-based matched controls. Conversely, patients with PTC had a 1.8-fold increased risk (P < .001) of developing CM. First- through fifth-degree relatives and spouses of patients with PTC or CM did not show a statistically significant increased risk. Eight patients with both cancer diagnoses had tissue specimens tested, of which 4 (50%) were found to be positive for the BRAF v600e mutation in either their PTC or CM specimen, and 3 (38%) were found positive in both. CONCLUSIONS AND RELEVANCE Patients with either PTC or CMhave an increased risk of developing the other cancer as a second primary malignant neoplasm. Tissue specimens from patients with both cancers show a high rate of BRAF v600e mutation. Translational studies are needed to better define the associated genetic predisposition between PTC and CM and to test the efficacy of and implementation techniques for treatment plans using BRAF mutation as a therapeutic target.