Increased commercial demand for monoclonal antibodies has resulted in an urgent need to establish alternative production systems. We previously developed a transgenic chicken bioreactor system that efficiently produced human cytokines in egg whites from genome-edited transgenic chickens. Here, we report on the application of this system to monoclonal antibody production. The genes encoding heavy and light chains of humanized anti-HER2 mAb linked by a 2A peptide sequence were integrated into the chicken ovalbumin locus using a CRISPR/Cas9 protocol. The knock-in hens produced a fully assembled humanized monoclonal antibody in their eggs. The mAb expression level in the egg white was 1.5–2.0 mg/mL by ELISA. Furthermore, the antigen binding affinity of the anti-HER2 mAb in egg white was estimated to be equal to that of the therapeutic anti-HER2 mAb (trastuzumab). In addition, antigen-specific binding by the egg white mAb was demonstrated by immunofluorescence against HER2 positive and negative cells.　These results indicate that the chicken bioreactor system can efficiently produce mAbs with antigen binding capacity and has the potential to be an alternative production system for commercial mAbs.