Background An increased risk of acute ischemic stroke is recognized among patients with cancer. However, the mechanism behind cancer‐related stroke is unclear. In this study, we determined the presence of associated venous thromboembolism and arterial thromboembolism and their clinical impact on patients with cancer‐related stroke. Methods and Results Patients with embolic stroke of undetermined source with or without cancer were evaluated for venous thromboembolism (deep vein thrombosis [ DVT ] and/or pulmonary embolism) and arterial thromboembolism by using Doppler sonography to determine the presence of lower‐extremity DVT and the microembolic signal of the symptomatic cerebral circulation, respectively. Infarct volume was determined by diffusion‐weighted magnetic resonance imaging. The multivariable linear regression and Cox proportional hazard analysis were used to investigate the effect of DVT and microembolic signal on infarct volume and 1‐year survival, respectively. Of 142 screened patients, 118 were included (37 with, 81 without cancer). Those with cancer had a higher prevalence of DVT or microembolic signal than did the noncancer group (62.2% versus 19.8%; P <0.001). Among patients with cancer‐related stroke, DVT was associated with a greater infarct volume in magnetic resonance imaging (beta, 13.14; 95% CI , 1.62–24.66; P =0.028). Presence of DVT (hazard ratio, 16.79; 95% CI, 2.05–137.75; P =0.009) and microembolic signal (hazard ratio, 8.16; 95% CI , 1.36–48.85; P =0.022) were independent predictors of poor 1‐year survival. Conclusions Patients with cancer‐associated embolic stroke of undetermined source have an elevated risk of associated venous thromboembolism and arterial thromboembolism, both of which have a significant negative impact on 1‐year survival. The results of this study may enhance our understanding of cancer‐associated stroke and improve risk stratification of patients with this disease. Clinical Trial Registration URL : https://www.clinicaltrials.gov/.Unique identifier: NCT 02212496