Background: Retrospective studies have reported an association between cancer and arterial thromboembolic event (ATE) risk. Objectives: We sought to confirm this in a prospective cohort with adjudicated outcomes. Methods: We evaluated participants enrolled in the REGARDS (REasons for Geographic and Racial Differences in Stroke) study with Medicare coverage for 365 days before their baseline visit (2003-2007). Medicare claims were used to identify new cancer diagnoses during follow-up. Using incidence-density sampling, participants who developed cancer were matched by age, sex, race, and education 1:4 to control participants who had not developed cancer. Participants were prospectively followed through 2015 for an expert-adjudicated ATE, defined as acute myocardial infarction or ischemic stroke. Cox regression was performed to evaluate the association between incident cancer and subsequent ATE. Results: In this analysis, 836 REGARDS participants with incident cancer were matched to 3339 control participants without cancer. In the 30 days after cancer diagnosis, 0.60% (n = 5) of the participants had an ATE; most of these events occurred near the time of cancer diagnosis. After adjustment for demographics, geographic region, and cardiovascular risk factors, compared to the noncancer controls, participants with incident cancer had an increased risk of ATE in the first 30 days after diagnosis (hazard ratio, 5.8; 95% confidence interval, 2.1-15.9). There was no association between cancer diagnosis and ATE beyond 30 days. Cancers with known metastases and types considered high risk for venous thromboembolism had the strongest associations with ATE. Conclusions: Incident cancer is associated with an increased short-term risk of ATE independent of vascular risk factors.