Background: Population aging and improving cancer survival rates are resulting in a growing population of older cancer survivors in the United States (US). As a result, there is an increasing need for longitudinal, population-representative data for interdisciplinary cancer research among older adults. The US Health Retirement Study (HRS) is an ongoing population-representative cohort of US adults over age 50 that contains rich interview and biomarker data on health during aging. Interviews have collected self-reported cancer diagnoses since 1998, but these self-reports have not been validated. We compared first incident cancer diagnoses self-reported in HRS interviews against diagnostic claims from linked Medicare records. Methods: We examined the validity of first incident cancer diagnoses self-reported in biennial HRS interviews from 2000 through 2016 against ICD-9 and ICD-10 diagnostic claim records among 8,242 HRS participants aged ≥65 with 90% continuous enrollment in fee-for-service Medicare, using the claim records as the gold standard. We calculated the sensitivity, specificity, and k for first incident cancer diagnoses (all cancers combined, excluding non-melanoma skin cancer, and each of bladder, breast, colorectal/anal, uterine, kidney/renal, lung/bronchus, and prostate cancers) cumulatively over the follow-up, and at each biennial study interview. Results: Self-reports of first incident cancer diagnosis (agnostic of site) between 2000 and 2016 had 73.2% sensitivity and 96.2% specificity against Medicare claims (k = 0.73). For site-specific self-reports, sensitivities ranged from 44.7% (kidney) to 75.0% (breast), and specificities ranged from 99.2% (prostate) to 99.9% (bladder, uterine, and kidney). Results were similar in sensitivity analyses restricting to individuals with 100% continuous fee-for-service Medicare enrollment and when restricting to individuals with at least 24 months of Medicare enrollment. Conclusions: Self-reported cancer diagnoses in the HRS have reasonable validity for population-based research on cancer and aging across cancer types. Apart from breast cancer, cancer site specific analyses will greatly benefit from the improved validity of self-report with Medicare claim linkage.