A male in his seventies presented with lung cancer in the right lower lobe. The surgically resected specimen revealed a pleomorphic carcinoma featuring an adenocarcinoma component with lepidic, acinar, and papillary patterns, alongside a spindle cell component spreading along the pulmonary artery wall, resembling intimal sarcoma. The spindle tumor cells were positive for keratins, TTF-1, napsin A, and vimentin, but negative for p40, CK14, desmin, alpha-smooth muscle actin, CDK4, and MDM2. This profile indicates that the spindle cells originated from the adenocarcinoma cells via epithelial-mesenchymal transition. The vascular spread of pleomorphic carcinoma, mimicking pulmonary artery intimal sarcoma, represents an extremely rare mode of tumor invasion, which could be termed "arterial carcinomatosis". Immunohistochemical analysis is crucial for distinguishing pleomorphic carcinoma from coexisting intimal sarcoma, as the latter's prognosis is markedly poor.