In this case report, a 7-year-old female Van cat of 2.3 kg weight, bred and given birth in the Van Cat Research Center, is presented. There was a two day history of inappetence and fatigue, along with lying without movement for long. Our physical examination showed an acute inflammation accompanied by swelling, severe pain and hardness in the left caudal thoracic mamma, in addition to a bruise starting from the nipple and extending to the rest of the tissue. The cat was diagnosed with mastitis. Escherichia coli and Staphylococcus simulans were identified after the microbiological analysis of the milk from the affected breast.