Neonatal mastitis is breast inflammation that occurs with or without abscess in the first 2 months of life. Mastitis is rare in the neonatal period. Inflammation is generally local and unilateral. If mastitis turns into abscess, fluctuating mass can be palpated. Ultrasonography is the preferred imaging method for the diagnosis of neonatal mastitis. We aimed to present a neonatal mastitis case evaluated in our clinic with the complaint of swelling in the left breast with ultrasonography (US) images.