Terminal deoxynucleotidyl transferase (TdT) is occasionally expressed in large B-cell lymphoma (LBCL), and this causes difficulty in differential diagnosis from B-lymphoblastic leukaemia/lymphoma (B-ALL/LBL). We reviewed 31 cases of TdT-positive LBCL and B-ALL/LBL, and their final diagnosis included 19 diffuse large/high-grade BCLs with MYC and BCL2 rearrangements (five DLBCL-MYC/BCL2, 14 HGBCL-MYC/BCL2), three DLBCL not otherwise specified (NOS), three HGBCL-NOS, four B-ALL/LBL, and two unclassifiable cases. TdT was variably expressed in all these cases, without any clear demarcation among different groups. Loss or partial loss of CD20 expression was seen in 13/17 DLBCL/HGBCL-MYC/BCL2, 2/3 HGBCL-NOS, and 2/2 unclassified, albeit not in DLBCL-NOS. Expression of BCL6 and/or MUM1 was seen in 3/4 B-ALL/LBLs and 2/2 unclassified. Next-generation sequencing revealed characteristic mutations associated with follicular lymphoma and its high-grade transformation in each DLBCL/HGBCL-MYC/BCL2, and also frequent variants in genes targeted by somatic hypermutation (SHM) in almost all DLBCL/HGBCL-MYC/BCL2, DLBCL-NOS, and HGBCL-NOS but one case. In contrast, such mutations were absent in B-ALL/LBL. There were no pathognomonic mutations in the two unclassifiable cases, although one showed a moderate level of somatic mutations in its rearranged IGHV. Furthermore, in three cases of TdT-positive HGBCL-MYC/BCL2, studies of previous or concurrent follicular lymphoma demonstrated their divergent evolution from an IGH::BCL2-positive cell population following acquisition of MYC translocation. In conclusion, mutation profiling analysis including the SHM target genes is highly valuable in the differential diagnosis between TdT-positive LBCL and B-ALL/LBL.