High-grade B-cell lymphoma, an aggressive form of Non-Hodgkin’s Lymphoma, is known as a double or triple hit lymphoma based on the presence of MYC and BCL2 without or with BCL6 genetic rearrangements, respectively. It carries a poorer prognosis, compared to other variants of B-cell lymphoma, and its management also differs which requires more intensive chemotherapy in contrast to the routine regimen. Terminal deoxynucleotidyl transferase (TdT), a marker of immaturity is commonly expressed in B-cell lymphoblastic leukemia or lymphoma (B cell ALL) which is absent in mature forms of B-cell lymphoma. The TdT is expressed in high-grade B-cell lymphoma; therefore, it poses a classification and management dilemma, which should be accurately differentiated from B-cell ALL and mandates molecular analysis. Herein, we report a case of a 52-year-old female with biopsy reported as high-grade B-cell lymphoma with TdT expression. She was referred for Fluor-deoxyglucose (FDG) Positron Emission Tomography-Computed Tomography (PET/CT) scan for staging in the absence of molecular analysis for B-cell ALL. It was diagnosed as lymphoma on FDG PET/CT based on its characteristic findings of extensive extranodal involvement of multiple organs with no significant lymphadenopathy establishing the incremental value of FDG PET/CT scan, which helped the clinician to arrive at a conclusion.