In recent years, protein drug development has gained momentum, and simple and facile controlled-release systems without loss of activity are required. Herein, we developed a sustained-release system for protein drugs by exploiting the “astringency” mechanism, namely insoluble precipitate formation by interacting with tannic acid. Tannic acid formed insoluble precipitates with various protein drugs, such as nisin, insulin, lysozyme, ovalbumin, hyaluronidase, and human immunoglobulin G, through hydrophobic interactions and hydrogen bonds. The lysozyme/tannic acid complex retained in vitro lytic activity. Precipitates of the insulin/tannic acid complex prolonged hypoglycemic effects without loss of activity after subcutaneous administration. The ovalbumin/tannic acid complex enhanced anti-ovalbumin antibody production induced by ovalbumin, which may be attributed to its sustained-release profile. Accordingly, tannic acid is useful as a simple and user-friendly drug delivery system for protein drugs.