A 35-year-old woman, gravida 5 para 4, at 20 weeks gestation, presented with fever, painful breast mass, painful nodular lesions on both lower limbs, and inflammatory arthralgias evolving for 2 months. No history of tuberculosis, sarcoidosis, breast trauma, or family breast disease was reported. Examination revealed multiple tender erythematous subcutaneous nodules on the legs and a large, irregular, infiltrated left breast mass without nipple discharge or retraction. Laboratory tests showed elevated C-reactive protein, normal blood count and negative autoimmune markers. Breast ultrasonography indicated mastitis (BIRADS 3), and core needle biopsy demonstrated non-caseating epithelioid and giant-cell granulomas with inflammatory infiltrate, without infection or malignancy. GeneXpert for Mycobacterium tuberculosis was negative. The diagnosis of idiopathic granulomatous mastitis associated with erythema nodosum and arthralgias was established. Cutaneous lesions improved rapidly after short-course oral corticosteroids, and a clinical follow-up strategy for the breast mass was adopted.