The Bluetongue virus (BTV) VP7 protein represents an important group-specific antigen that can serve as a basis for diagnostic tests. Here, we report the generation of a novel BTV group-specific monoclonal antibody (Mab; herein named 4H7) that recognizes a conformational epitope in the VP7 protein. We used a phage-displayed peptide screen and site-directed mutagenesis to define the VP7 amino acid residues that most strongly contribute to the conformational epitope recognized by Mab 4H7. Amino acid residues at positions 175, 185, 186, and 278 of the BTV VP7 protein strongly contributed to Mab 4H7 binding. These key amino acid residues are conserved among all BTV serotypes, whereas related Orbiviruses possess at least one amino acid substitution at these positions. We developed a competitive enzyme-linked immunosorbent assay (c-ELISA) using Mab 4H7 and recombinant BTV VP7 protein to detect serum antibodies against this BTV group-specific VP7 epitope. The c-ELISA was used to screen 833 clinical samples collected from animals in three provinces of China. BTV seroprevalence in the three provinces ranged from 25.42 to 47.45 %. This work provides the foundation for a new diagnostic c-ELISA that can be further applied to BTV surveillance activities and informs our understanding of the structure of the BTV VP7 protein.