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Summary Inventions into chitosan oligosaccharide (COS) derivatives over recent decades have hugely broadened their application in food safety. Herein, monomethyl fumaric acid (MFA) was grafted on the OH position of COS to synthesise new COS‐O‐MFA derivatives and evaluated their in vitro antibacterial properties. The degrees of substitution (DS) of the derivatives yielded almost 15% with the range of 0.093 to 0.260. The in vitro antibacterial properties of the new COS‐O‐MFA1‐3 derivatives against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) were significantly enhanced concerning COS. Based on the finding, the introduction of MFA and increasing DS have a great impact on the enhancement of antibacterial properties of the COS‐O‐MFA1‐3 derivatives. This work proves that COS‐O‐MFA1‐3 derivatives can be potential novel antibacterial agents and also open up a new avenue to improve their antibacterial attributes.