Monoclonal antibody therapy is one of the most promising approaches for effective influenza control. In this study, we evaluated the antiviral activity of exogenous mRNA-encoded single-chain variable fragment (scFv) antibodies, which are capable of binding viral antigens inside the cell with high affinity. Two influenza virus proteins, hemagglutinin (antibody FI6) and nucleoprotein (antibody 2/3), were chosen as targets. Each scFv encoded by mRNA was produced in two variants: one containing a signal peptide (SP) to direct secretion into the extracellular space (scFv-SP) and one lacking the signal peptide (scFv-WO) for cytosolic localization and function. These variants showed distinct intracellular localization patterns: scFv-SP localized to regions characteristic of the endoplasmic reticulum and the Golgi complex, whereas scFv-WO was distributed diffusely throughout the cytoplasm. mRNAs encoding scFv-FI6-SP, scFv-2/3-SP, and scFv-2/3-WO exhibited antiviral activity against influenza A virus in vitro. The scFv-FI6-SP mRNA showed the strongest antiviral effect, reducing viral load by approximately tenfold compared to the control. For influenza B virus, both  scFv-2/3 mRNA variants, with and without the signal peptide,  reduced viral load by an average of 50%. These findings highlight the antiviral potential of intracellular antibodies and point to new opportunities for targeting viral components that are not accessible to conventional antiviral therapies.