Influenza viruses continue to pose a significant threat to human and animal health. However, the limited number of licensed antivirals is increasingly compromised by drug resistance driven by high mutation rates. This highlights the urgent need for broad-spectrum therapeutics with novel mechanisms of action. Nanobodies, a new generation of antibody drugs, have great potential in the treatment of influenza virus infections. Neuraminidase (NA) mediates the sialic acid cleavage required for viral release, and its genetic drift is generally slower than that of hemagglutinin (HA) in influenza viruses, making it an attractive target for broad-spectrum antiviral and vaccine development. To construct a phage display nanobody library targeting NA, an alpaca was immunized with the NA protein of the H9N2 influenza virus. A specific nanobody, designated F4, was subsequently screened from the immune library. The nanobody was engineered into an Fc-fused nanobody, F4-Fc, which exhibited inhibitory activity against multiple influenza A and B viruses in vitro and provided robust prophylactic and therapeutic protection against influenza A and influenza B virus infections in vivo. Mechanistically, F4-Fc inhibits NA enzymatic activity and mediates antibody-dependent cellular cytotoxicity. In conclusion, F4-Fc demonstrates prophylactic and therapeutic efficacy against influenza A and B viruses, representing a promising antiviral drug candidate for influenza virus infection.