INTRODUCTION Idiopathic granulomatous lobular mastitis (IGLM) is an inflammatory disease of the breast with an obscure etiology. It occurs mainly in women of reproductive age, and the lesion mimics carcinoma of the breast both clinically and radiologically. CASE PRESENTATION We present the case of a 29-year-old female who visited our hospital in Kancheepuram, Tamil Nadu, with a 4 × 3 cm lump in the upper outer quadrant of her left breast. The clinical and radiological findings were indicative of a malignant lesion; however, fine-needle aspiration cytology (FNAC) revealed features of granulomatous mastitis, and the subsequent histology of the excised lump confirmed the diagnosis of IGLM. CONCLUSIONS IGLM should be considered as one of the differential diagnoses when granulomas are encountered in breast FNAC and biopsy. A definitive diagnosis of IGLM can be made by identifying its characteristic histomorphology and ruling out other causes for granulomatous inflammation. An exact diagnosis is essential since the treatment for different granulomatous conditions of the breast varies.