BACKGROUND: This study aims to compare the seventh and eighth editions of the American Joint Commission on Cancer/Union for International Cancer Control (AJCC/UICC) tumor, node, metastasis staging system for patients with papillary thyroid cancer (PTC) in two national patient cohorts. METHODS: Adult PTC patients undergoing surgery were selected from the Surveillance, Epidemiology and End Results (SEER) program (2004-2012) and the National Cancer Database (2004-2012). Staging criteria for the seventh and eighth AJCC/UICC editions were applied separately to each cohort. Survival probabilities were estimated using the Kaplan-Meier method. Multivariable Cox proportional hazards models were used to estimate the association of stage with survival in both settings. The Akaike information criterion was used to assess model performance. RESULTS: About 23% of patients were downstaged from the seventh to the eighth edition in SEER, while 24% were downstaged in the National Cancer Database. Disease-specific survival (DSS) and overall survival (OS) were significantly related to stage at diagnosis when using both the seventh and eighth editions of the AJCC/UICC staging system (p < 0.001). Patients classified into higher stages (III and IV) in the eighth edition showed a worse prognosis than those classified into similar stages in the seventh edition. After adjustment, PTC stages as defined by both editions were significantly associated with DSS and OS. With respect to both DSS and OS, the eighth edition PTC model appeared to be a better fit to the data (smaller Akaike information criterion values) compared to the seventh edition. CONCLUSION: Based on these large contemporary national cohorts, the eighth edition AJCC/UICC tumor, node, metastasis classification for PTC is superior to the seventh edition for predicting survival.