Background: A 22 years old young married female presented with complaints of dry cough, breathlessness, decreased appetite and low–grade fever for 2 months. No history of chest pain, hemoptysis and loss of weight. The patient was on treatment for infertility for the last 3 months. Examination revealed trachea shifted to the left side, stony dull note on percussion, reduced intensity of breath on the right side of the chest. A diagnosis of right–sided pleural effusion –? tubercular etiology was made. Pleural fluid analysis revealed exudative effusion with lymphocytic predominance. Therapeutic thoracocentesis was done along with CAT–1 ATT. Repeat imaging presence of a mass lesion in the right pleural cavity. The patient was further evaluated with Mediastinal cyst fluid analysis, biopsy, PETCT. The patient underwent right pleural cyst excision. The immunoprofile of mediastinal solid mass is consistent with “SYNOVIAL SARCOMA”. As PETCT was normal, the patient was not started on chemo or radiotherapy and was advised to review on a regular basis. Conclusion: Primary synovial sarcoma is a very rare malignancy of lung, 0.5% of overall lung cancers. The most appropriate management is surgical excision. Due to high rates of recurrence a very close follow up is needed. Highly aggressive and with poor prognosis.