During clathrin-mediated endocytosis, adaptor proteins recognize specific internalization signals on cargo receptors, either recruiting cargos into clathrin-coated pits (CCPs) or initiating clathrin-coat assembly around the cargo molecules. Here, we identify epsin 1, a clathrin-, ubiquitin-, and phospholipid-interacting protein, as a cargo-specific adaptor for influenza virus entry through the clathrin-mediated pathway. Using live-cell imaging to monitor the entry of individual virus particles, we observed recruitment of epsin 1 to the binding sites of influenza viruses in synchrony with the assembly of CCPs. Epsin 1 knockdown by siRNA significantly inhibited the clathrin-mediated endocytosis of the influenza virus and caused the majority of the virus particles to enter through a clathrin-independent pathway. The same treatment did not affect the entry of several classical ligands for clathrin-mediated endocytosis, including transferrin, LDL, and EGF. Overexpression of the dominant-negative epsin 1 mutant lacking the ubiquitin-interaction motifs nearly completely blocked the clathrin-mediated entry of the influenza virus without affecting transferrin uptake. These results suggest that epsin 1 functions as a cargo-specific adaptor for the clathrin-mediated entry of the influenza virus.