Influenza A viruses, particularly H1N1, pose a significant public health threat, highlighting the continued need for novel prophylactics and therapeutics. Monoclonal antibodies are promising, but conventional IgGs face challenges in respiratory delivery due to their large size. This study presents A1-B7, a C-type single-domain antibody (C-sdAb) based on a scaffold derived from a human IgG1 CH2 domain, targeting the hemagglutinin (HA) of H1N1 influenza virus. A1-B7 potently neutralizes multiple H1N1 subtypes in vitro. Both prophylactic intranasal administration and therapeutic aerosol inhalation of A1-B7 confer complete protection and reduce live virus in the lungs to undetectable levels. Structural prediction and epitope mapping reveal that A1-B7 binds a conserved stem-region epitope, engaging the HA2 A-helix and a hydrophobic groove. A1-B7 can block low-pH induced conformational change of HA. These findings establish A1-B7 as a promising candidate for further development as a convenient, respiratory tract delivered countermeasure against influenza.