Background: Knowledge of the prevalence of thromboemboli and the associated hemostatic status in dogs with carcinoma or sarcoma is unknown and might allow earlier intervention. Objectives: Estimate prevalence of thromboemboli and their association with hemostatic changes in dogs with carcinomas or sarcomas; estimate predictive values of hemostatic variables for thromboembolic disease in tumor-bearing dogs. Animals: Thirty-two dogs with sarcoma, 30 with carcinoma, 20 healthy age-controlled dogs. Methods: Prospective cross-sectional study. A hemostasis panel (platelet concentration, thromboelastography, fibrinogen and D-dimer concentration, factor X, VII and antithrombin activity) was performed in all dogs. Tumor-bearing dogs underwent complete post mortem and histopathological evaluation. Comparisons between healthy dogs and tumor-bearing dogs with and without intracavitary hemorrhage; and tumor-bearing dogs with and without microthrombi were analyzed. Results: Thromboembolic disease was identified in 32/62 (52%, 95% CI: 39%-65%) tumor-bearing dogs. Microthrombi were identified in 31/62 (50%, 95% CI: 37%-63%) dogs, 21/31 (68%, 95% CI: 49%-83%) had exclusively intra-tumoral microthrombi, 10/31 (32%, 95% CI: 17%-51%) had distant microthrombi. Macrothrombi were identified in 3 tumor-bearing dogs. Hemostatic changes potentially consistent with overt and non-overt disseminated intravascular coagulation were identified in some tumor-bearing dogs. D-dimer concentrations were significantly higher (P =.02) and platelet concentration significantly lower (P =.03) in tumor-bearing dogs with microthrombi compared to tumor-bearing dogs without microthrombi. D-dimer concentration above 500 ng/mL was 80% sensitive and 41% specific for the prediction of microthrombi presence. Conclusion: The high microthrombi prevalence and concomitant hemostatic dysfunction in dogs with carcinomas or sarcomas has not previously been reported, though the clinical importance is unknown. Increased D-dimer concentration might increase suspicion of microthrombi.