The etiology of complicated pleural effusion can be vast. We present a unique case of an unsuspected metastatic endometrial stromal sarcoma (ESS) in an asymptomatic patient with an incidentally found complicated pleural effusion. A 69-year-old female with no pertinent past medical history was referred to pulmonology for an effusion noted on a routine chest X-ray. Her surgical history was significant for a hysterectomy. At the time of evaluation in the pulmonology clinic, the patient was asymptomatic with stable vital signs. Computed tomography of her chest showed a complex pleural effusion which was drained by cardiothoracic surgery. Fluid analysis results were positive for estrogen and progesterone receptor-positive mesenchymal tumor. Follow-up imaging was negative for any other metastasis. Appropriate management and drainage of this asymptomatic pleural effusion resulted in the diagnosis of a rare malignancy. Given the good clinical prognosis of mesenchymal tumors, the patient was appropriately treated and doing well.We present the case of a patient who was found to have a rare malignancy rather than a benign chronic pleural effusion, as previously suspected. This neoplasm represented a metastatic ESS, especially in this patient’s setting of a hysterectomy.