Influenza A virus is a segmented, negative-sense RNA virus. Since the early 2020s, H5 clade 2.3.4.4b viruses have spread widely across Europe, Africa, and Asia, affecting wild birds and poultry. Costa Rica reported its first H5 clade 2.3.4.4b avian influenza virus (AIV) case on 19 January 2023. This study describes an outbreak in backyard chickens and ducks. Initial serum samples collected on 24 January showed three chickens negative for AIV, while one duck tested positive by ELISA and agar gel immunodiffusion (AGID). During a second visit on 27 January, three of four chicken sera collected tested positive by ELISA and AGID. Tissue samples were positive for influenza A by qRT-PCR. Next-generation sequencing recovered five of the eight viral genomic segments, and the hemagglutinin cleavage site sequence (REKRRKR↓G) confirmed a highly pathogenic avian influenza virus (HPAIV) H5 strain. The samples were submitted to the National Veterinary Services Laboratories for confirmation. Serological testing showed reactivity to North American low pathogenic H5 antigens, and qRT-PCR amplified influenza A and N1 genes. Virus isolation and next-generation sequencing (NGS) of all eight viral genome segments were successfully performed at the WHO Collaborating Centre at St. Jude Children’s Research Hospital (SJCRH).