In March 2013, the first human case of zoonotic H7N9 avian influenza virus (AIV) infection was reported in China. This virus has been circulating in domestic poultry in China while mutating to highly pathogenic AIV (HPAIV) since 2017, which caused human infections and poultry outbreaks. In 2019, a novel reassortant H7N2 HPAIV, A/chicken/China/SJZ1/2019(SJZ1), was isolated from H7-Re2-vaccinated layers. We analyzed the genetic, pathogenic, and antigenic characteristics of SJZ1. Analysis of the entire SJZ1 genomic sequence revealed that it comprised at least two different sources; the PB2, PB1, PA, HA (H7), M, and NS segments of SJZ1 were directly derived from H7N9 AIVs, whereas the NA (N2) and NP segments of SJZ1 were derived from H9N2 AIVs. Experimental infection revealed that SJZ1 was highly pathogenic in chickens but not in ducks. SJZ1 was shed from and replicated in chickens and ducks. Hemagglutination-inhibition assay and challenge test indicated that SJZ1 exhibited rapid antigenic drift and distinct antigenicity relative to the H7-Re2 vaccine strain, which provides poor protection for SJZ1. Our study reports the emergence of a new reassortant of H7N2 AIV with novel viral characteristics and warns of the challenge we still face to control the zoonotic H7N9 AIVs and their reassortants.