Renal vein thrombosis (RVT) is rare and primarily observed in children with severe dehydration or in adults in a hypercoagulable state. This diagnosis is rarely considered when it occurs in adults. We report a case of a young man who had weight loss of 8 kg in 2 weeks accompanied by dehydration with excessive exercise, and he developed a right RVT with a pulmonary thromboembolism. The man had a 3-year history of essential hypertension and was admitted to the hospital because of severe right-flank pain. A RVT and pulmonary thromboemboli were visualized by computed tomography. No abnormal results were observed on coagulation tests, and no evidence of malignancy was found. We concluded that the RVT and pulmonary thromboembolism were induced by dehydration. Even though the patient was an adult, rapid weight loss with dehydration may cause RVT and unusual thromboembolic events must be suspected to avoid a delay in the diagnosis.