Purpose <p>Assessing the prognostic value of ¹⁸F-fluorodeoxyglucose (FDG) PET/CT in patients with pulmonary invasive mucinous adenocarcinoma (IMA).</p> Methods <p>This dual-centre retrospective study included patients diagnosed with pulmonary IMA between January 2010 and August 2020. The patients were categorized into three groups based on CT morphology: solitary pulmonary nodule (SPN), pneumonic, and multifocal types. FDG avidity of the primary lesion was visually assessed using the mediastinal blood pool as a reference. Disease-free survival (DFS) was analysed in patients who underwent curative surgical resection.</p> Results <p>A total of 139 patients (mean age: 69.1 years) were included. Most patients had SPN-type tumors (63.3%), of which 60.2% were FDG-avid, whereas all patients with pneumonic-type (18.0%) were FDG-avid. DFS analysis was performed in 78 surgically treated patients, with recurrence observed in 18 cases. Univariate analysis identified T stage, nodal involvement, CT imaging phenotype, visceral pleural invasion (VPI), and FDG avidity as significant prognostic factors. In a multivariate analysis, CT imaging phenotype, VPI, and FDG avidity remained independent prognostic factors. When patients were stratified into three groups—non-FDG-avid SPN-type, FDG-avid SPN-type, and pneumonic-type—Kaplan-Meier analysis demonstrated a significantly longer DFS in non-FDG-avid SPN-type patients than in the other groups. The median DFS was not reached for non-FDG-avid or FDG-avid SPN-type groups, whereas it was 21.0 months for patients in the pneumonic-type group.</p> Conclusion <p>Utilization of FDG PET/CT, particularly when combined with CT findings, may enhance the prognostic stratification of patients with curatively resected IMA of the lung, as visual FDG avidity is associated with worse prognosis.</p>