Skin lymphomas are less common and subcutaneous panniculitis-like T-cell lymphoma (SPTCL) is a fairly rare subtype of primary cutaneous lymphoma. Skin lymphomas involve subcutaneous adipose tissues with no involvement of lymph nodes. Diagnosis of these cases is generally a challenge to clinicians. These cases present with fever, weight loss, and local discomfort in the region of involvement of subcutaneous tissues and sometime with skin eczema and rashes. Positron emission tomography/computed tomography (PET/CT) scan can guide in determining the extent of involvement being whole-body imaging and can guide the site of biopsy and can help to prevent misdiagnosis. It also helps in correct and early diagnosis and successful treatment. We report a case of a young adult who presented with pyrexia of unknown origin in which PET/CT scan revealed mildly fluorodeoxyglucose-avid diffuse subcutaneous panniculitis involving the whole body, trunk, and extremities. Biopsy was taken from the most appropriate site according to the PET/CT scan report and reported as SPTCL.