Background: Presentation of granulocytic sarcoma (GS) as an effusion is very rare and most cases are misdiagnosed as lymphoma infiltration. Detailed descriptions of cytological features of GS in an effusion have not been published to date. Case Report: We present the case of a 35-year-old male who presented clinically with a mediastinal mass, peripheral lymphadenopathy and pleural/pericardial effusion. His effusion cytology showed an atypical morphology. Immature cells with marked nuclear indentation/convolution, fine nuclear chromatin, conspicuous nucleoli and a moderate amount of cytoplasm were seen, without any granularity. GS was suspected based on the abnormal nuclear contours and fine nuclear chromatin, and was confirmed by lymph node fine needle aspiration and biopsy with immunocytochemistry. Conclusion: Abnormal nuclear contours are a significant marker in effusion cytology and provide an important morphological clue to the diagnosis of a myeloid malignancy in addition to fine nuclear chromatin especially in the absence of cytoplasmic granularity. In this case, these features helped to suspect the diagnosis despite a clinical scenario suggestive of non-Hodgkin’s lymphoma.