A 17-year-old spayed female Shih Tzu dog, weighing 5.0 kg, presented with frequent coughing and respiratory distress. Blood tests revealed mild thrombocytosis, and thoracic ultra-sonography and radiography confirmed a significant amount of pleural effusion. However,the thoracic radiographs showed no radiopaque nodules or interstitial patterns indicative ofthoracic tumors. Thoracentesis was performed to relieve effusion-induced thoracic pressure, yielding a hemorrhagic serosanguinous pleural fluid. The cytological analysis of this fluid re-vealed mesothelial cells, supporting the clinical diagnosis of mesothelioma in situ. To address the patient’s clinical symptoms, an aggressive management approach was implemented withchest tube placement to address recurrent pleural effusion after initial thoracentesis. Duringtreatment, the patient exhibited stable health and adapted well to daily life. To the best of our knowledge, this is the first reported case of mesothelioma in situ with hemorrhagic malignant pleural effusion in South Korea. Using a chest tube as an aggressive treatment successfullyalleviated dyspnea symptoms and provided symptomatic relief in a patient with mesothelioma in situ.