Some nanozyme-based detection methods for antioxidants often rely on the direct decolorization of colorimetric substrates by corresponding antioxidants, which weakens the role of nanozymes. Here, CeO nanozymes exhibiting inherent oxidase-like activity were prepared via a one-pot hydrothermal technique. Ergothioneine (ERG), a natural antioxidant, was found to induce the true inhibition of the oxidase-like activity. Unlike other antioxidants including ascorbic acid, gallic acid and glutathione, ERG could really inhibit the oxidase-like activity of CeO nanozymes, instead of directly reducing colorimetric substrates. Further kinetic analysis revealed that ERG-induced inhibition is reversible and noncompetitive. This discovery highlights the unique behavior of ERG and provides an important mechanistic understanding of nanozyme inhibition by antioxidants. Leveraging the inhibitory effect, a novel colorimetric sensing platform for ERG was established with a low limit of detection (0.21 μM), excellent reusability, good storage stability and high accuracy, making it a simple, sensitive and rapid method for assessing ERG levels in functional foods. This study not only establishes a framework for investigating the interactions between nanozymes and small molecules but also paves a new pathway for expanding the applications of nanozymes in food and agricultural biosensing.