Objectives <p>To evaluate the diagnostic performance of [¹⁸F]fluorodeoxyglucose positron emission tomography/computed tomography ([¹⁸F]FDG PET/CT) in detecting primary cutaneous lesions, lymph node involvement, and extranodal non-lymphatic disease in patients with primary cutaneous lymphoma (PCL).</p> Methods <p>We retrospectively analyzed 84 diagnosed PCL patients who underwent [¹⁸F]FDG PET/CT between 2008 and 2023. The intensity of FDG uptake in primary cutaneous lesions, lymph nodes, and extranodal non-lymphatic sites was assessed. Diagnostic performance, including sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and area under the receiver operating characteristic curve (AUC) were evaluated using histopathology or long-term clinical follow-up as reference standards.</p> Results <p>Patients were categorized into mycosis fungoides/Sézary syndrome (MF/SS, <i>n</i> = 30) and non-MF/SS subtypes (<i>n</i> = 54). PET/CT detected primary skin lesions in 76.7% of MF/SS patients (median SUVmax: 3.5, range: 1.2–19.8) and 92.6% of non-MF/SS patients (median SUVmax: 6.35, range: 1.0–33.0). Lymph node involvement was pathologically confirmed in 5 MF/SS and 24 non-MF/SS patients, with ROC analysis identified optimal SUVmax thresholds of 8.5 for MF/SS (AUC = 0.990, <i>p</i> = 0.001) and 4.5 for non-MF/SS (AUC = 0.976, <i>p</i> = 0.000) respectively. Extranodal non-lymphatic lesions were detected in 16.7% (5/30) of MF/SS and 57.4% (31/54) of non-MF/SS patients, with median SUVmax values of 5.1 (range 3.5–19.3) and 3.8 (range 0.8–19.2), respectively.</p> Conclusions <p>PET/CT demonstrated higher sensitivity for detecting skin lesions in non-MF/SS than in MF/SS. Subtype-specific SUVmax thresholds 8.5 for MF/SS and 4.5 for non-MF/SS, significantly improve diagnostic accuracy for lymph node involvement. PET/CT also accurately detected extranodal disease, emphasizing its value in staging PCL.</p>