Wastewater surveillance is an important tool in the surveillance of emerging pathogens and has been leveraged during the highly pathogenic avian influenza (HPAI) A(H5N1) virus outbreak in cattle and poultry in the United States. Interpretation of avian influenza A(H5) subtype detections in wastewater requires an understanding of human and animal contributors to the sewershed because current testing does not distinguish between human and animal sources. Potential animal contributors include wild birds, farms with poultry or dairy cattle outbreaks, and dairy processing facilities. Retrospective analysis of 551 influenza A virus–positive wastewater surveillance samples from 20 sites in Oregon during September 15, 2021–July 11, 2024, revealed 21 avian influenza A(H5) subtype detections across 12 communities. Avian influenza A(H5) subtype detections in wastewater began approximately 6 weeks before Oregon’s first HPAI outbreak in domestic poultry, 7 weeks before Oregon’s first avian influenza A(H5) detection in wild birds, and 2 years before the first HPAI A(H5N1) outbreak in dairy cattle in the United States (Oregon has not detected HPAI A(H5N1) in dairy cattle or milk). No association was found between detection of avian influenza A(H5) in a community’s wastewater and history of an HPAI A(H5) outbreak among poultry in the county or presence of dairy processing facilities or dairy farms within the sewershed. Avian influenza A(H5) was detected most frequently in two communities with important wild bird habitats. Animal inputs, including from wild birds, should be considered when interpreting avian influenza A(H5) subtype detections in wastewater.