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Solitary extramedullary plasmacytoma of the kidney is an exceptionally rare plasma cell neoplasm and may closely mimic renal cell carcinoma on imaging. A woman in her late 50s was incidentally found to have a left renal mass during evaluation for non-specific abdominal discomfort. Cross-sectional imaging revealed a multiloculated cystic renal lesion with enhancing walls, raising concern for primary renal malignancy. CT-guided biopsy demonstrated sheets of plasma cells with kappa light-chain restriction. Comprehensive evaluation, including serum studies, positron emission tomography–CT and bone marrow biopsy, showed no evidence of systemic multiple myeloma, confirming the diagnosis of solitary renal plasmacytoma. The patient was treated with definitive external beam radiotherapy and achieved complete radiological resolution, with no evidence of progression on follow-up. This case highlights an important diagnostic pitfall and emphasises the value of early tissue diagnosis to avoid unnecessary nephrectomy and allow curative local therapy.