A 70-year-old woman was referred to our hospital due to a mass lesion detected in the left lower lobe of the lung field on chest computed tomography (CT). A bronchoscopy was performed, and she was diagnosed with lung adenocarcinoma, cT2bN0M0 Stage IIA, and partial thoracoscopic lung resection of the left lower lobe was performed due to extensive pleural dissemination in the lower left lobe. Histological examination of the resected tumor showed that clear columnar cells with abundant mucus had proliferated to form a small gland duct. The tumor cells yielded CK7 (+), CK20 (-), CDX2 (+), TTF-1 (-), and Napsin A (-) on immunohistological examination. Upper and lower endoscopy revealed no gastrointestinal tract tumors. Therefore, we finally diagnosed lung adenocarcinoma with a signet ring cell carcinoma-like morphology. She received carboplatin + pemetrexed + pembrolizumab combination therapy as a first-line treatment.