Abstract Background: The US Health Retirement Study (HRS) is an ongoing population-representative cohort of US adults ages >50 with rich data on health during aging. Self-reported cancer diagnoses have been collected since 1998, but they have not been validated. We compared self-reported cancer diagnoses in HRS interviews against diagnostic claims from linked Medicare records. Methods: Using HRS–Medicare linked data, we examined the validity of first incident cancer diagnoses self-reported in biennial interviews from 2000 to 2016 against ICD-9 and ICD-10 diagnostic claim records as the gold standard. Data were from 8,242 HRS participants ages ≥65 with 90% continuous enrollment in fee-for-service Medicare. We calculated the sensitivity, specificity, and κ for first incident invasive cancer diagnoses (all cancers combined, and each of bladder, breast, colorectal/anal, uterine, kidney, lung, and prostate cancers) cumulatively over the follow-up and at each biennial study interview. Results: Overall, self-reports of first incident cancer diagnoses from 2000 to 2016 had 73.2% sensitivity and 96.2% specificity against Medicare claims (κ = 0.73). For specific cancer types, sensitivities ranged from 44.7% (kidney) to 75.0% (breast), and specificities ranged from 99.2% (prostate) and 99.9% (bladder, uterine, and kidney). Results were similar in sensitivity analyses restricted to individuals with 100% continuous fee-for-service Medicare enrollment and when restricted to individuals with at least 24 months of Medicare enrollment. Conclusions: Self-reported cancer diagnoses in the HRS have reasonable validity for use in population-based research that is maximized with linkage to Medicare. Impact: These findings inform the use of the HRS for population-based cancer and aging research.