Pulmonary colloid adenocarcinoma is a rare subtype of lung adenocarcinoma that accounts for less than 1% of all cases and is characterized by abundant extracellular mucin and sparse tumor cells. It often mimics infectious lung diseases, which makes preoperative diagnosis difficult. A 44-year-old woman presented with fever, pleuritic chest pain, and shortness of breath, with leukocytosis, elevated C-reactive protein, and procalcitonin. Radiology showed an 11 cm septated cystic lesion in the right lower lobe with lobar consolidation and a small pleural effusion, suspicious for a lung abscess. Antibiotics and percutaneous drainage did not work, so surgical resection followed. Histopathology confirmed colloid adenocarcinoma with invasion of the visceral pleura, without nodal metastases. Sequencing revealed concurrent KRAS and TP53 mutations plus a truncated variant of AXIN2. After adjuvant chemotherapy, the patient has no recurrence for more than 3 years.