Granulomatous mastitis is a rare benign disease that typically occurs in parous women. Some reports have described cases of erythema nodosum appearing following granulomatous mastitis, which is often treated with steroids. Here, we report a case of granulomatous mastitis with erythema nodosum successfully treated via drainage only, which may have been caused by the higher plasma cortisol levels observed during pregnancy. Although mastitis is rare during pregnancy, the current case suggests that granulomatous mastitis should be considered in pregnant women with treatment-resistant mastitis, especially in those with erythema nodosum and a history of birth. Furthermore, patients with granulomatous mastitis may not require prednisolone treatment during pregnancy, which may help in preventing steroid-associated conditions such as infections and gestational diabetes mellitus.