Diagnosis of anomalous intrathoracic lesions may be challenging and require a multidisciplinary approach. We present a case of granulomatosis with polyangiitis (GPA) clinically and radiologically mimicking metastatic lung cancer with a bilateral pulmonary mass, mediastinal and cervical lymph node involvement, and pleural effusion. Surgical biopsy of the thoracic lesion revealed necrotic granulomatous inflammation, and the final diagnosis was subsequently confirmed by kidney biopsy and biochemical parameters. This case illustrates how comprehensive diagnosis secures timely and relevant treatment. Systemic vasculitis may be one of the key differential diagnoses in patients with multiorgan involvement, especially with pattern-mimicking lung cancer.