ABSTRACTA biobased polymer derived from cashew nut shell liquid (CNSL) as a renewable resource was investigated for use as an antibacterial material. CNSL is a mixture of aromatics containing cardanol as the main component and cardol and 2‐methylcardol as minor components. CNSL composition analyses showed that the minor components (i.e., cardol and 2‐methylcardol) in CNSL had higher contents of unsaturated structures than cardanol. These higher unsaturated contents promoted the thermal polymerization in the preparation of an epoxy CNSL prepolymer (ECNP). The biobased polymer film was fabricated by the reaction of amine compounds and ECNP without any organic solvent. The ECNP film took less than 2.0 h to reach a hardened dry condition at room temperature because of the crosslinking reaction between epoxy and amine groups. The antibacterial activities of the biobased polymer against Escherichia coli and Staphylococcus aureus were evaluated. CNSL showed antibacterial activity against S. aureus, whereas epoxy CNSL and ECNP alone showed no significant antibacterial activity against E. coli or S. aureus. This indicated that the antibacterial activity was based on the phenolic and catechol hydroxyl groups of CNSL. In addition, a biobased polymer film derived from CNSL and diamine showed antibacterial activity against both E. coli and S. aureus, even with alcohol conditioning. This suggested that the antibacterial activity was certainly fixed in the structure of the ECNP‐based polymers after the standard antisepsis treatment in medical facilities. Therefore, this biobased polymer could be useful in antibacterial materials as a coating and resin for health care applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42725.