Chicken infectious bursal disease (IBD) is an acute, highly contagious, lethal, and immunosuppressive disease caused by the infectious bursal disease virus (IBDV). The newly emerged novel variant IBDV (nVarIBDV) and the continuously circulating very virulent IBDV (vvIBDV) are the two predominant epidemic strains that pose a threat to the poultry industry in several countries, including China. However, on-site rapid detection methods for distinguishing nVarIBDV from vvIBDV remain lacking. In this study, a neutralizing monoclonal antibody (mAb), 2F10, capable of differentiating nVarIBDV from vvIBDV was successfully developed for the first time. The antigenic epitope recognized by mAb 2F10 is conformation-dependent, with residue 318 of VP2 being the key determinant of its specificity. Significant differences in hydrogen bonding, hydrophobic interactions, and salt bridges at the VP2-mAb interface-caused by the consistent substitution of aspartic acid (in nVarIBDV) and glycine (in vvIBDV) at residue 318-constitute the core mechanism by which mAb 2F10 selectively recognizes nVarIBDV but not vvIBDV. Further data confirm that mAb 2F10 can be utilized to develop a competitive ELISA for the specific detection of nVarIBDV antibodies. This study not only contributes to a deeper understanding of the antigen structure and immune evasion mechanism of nVarIBDV but also provides a valuable tool for molecular tracing and differential detection of this novel variant.