In June 2023, Poland experienced an unprecedented outbreak of highly pathogenic avian influenza (HPAI) H5N1 virus infections in cats, raising concerns about the role of cats in HPAI H5N1 virus ecology and potential public health implications. We describe the clinical course and molecular findings in a cat designated as cat No. 14 during this outbreak, with emphasis on the viral load detected in various organs and tissues. The cat was initially examined for recurrent dermatological problems, although the owner also reported reduced activity. Over the following five days, the disease rapidly progressed, with systemic and neurological signs unresponsive to treatment. Blood analyses revealed no major haematological abnormalities, whereas serial serum biochemistry showed progressive abnormalities. Due to the poor prognosis, the cat was humanely euthanized five days after the initial veterinary examination. Tissue samples from major internal organs and lymph nodes, together with tracheal and rectal swabs and bile, were collected for real-time RT-PCR. Viral RNA was detected in all analysed samples except the intestines, mesenteric lymph nodes, rectal swab, and bile, with the highest viral loads observed in the cerebellum and brain. This case provides molecular evidence of widespread HPAI H5N1 viral RNA distribution across tissues, complemented by detailed haematological and biochemical findings, further characterizing the systemic nature of infection.