Tea contains various antioxidant compounds, including polyphenols, catechins, theaflavins, theasinensins, and flavonoids. Among these, epigallocatechin gallate (EGCG) is a crucial antioxidant recognized for its potent bioactivity. This study presents the synthesis of a highly selective Cu-PyC NH4+-based metal-organic framework (MOF) nanozyme that exhibits catecholase-like activity to assess the antioxidant capabilities of EGCG. The developed nanozyme demonstrates robust stability and specificity in oxidizing 3,5-Di-tert-butylcatechol (3,5-DTBC), showcasing unique catecholase activity distinct from typical oxidase nanozymes. Furthermore, this nanozyme displays exceptional efficacy, sensitivity, and selectivity in targeting EGCG, enabling accurate quantification of EGCG levels in commercial tea products via UV-spectroscopy. The assay exhibits a linear response within the EGCG concentration range of 0.5–125 µM, with a detection limit of 0.83 µM, alongside excellent reproducibility and stability. These findings suggest that this nanozyme offers a promising approach for precisely evaluating antioxidants, with significant implications for the food and beverage industry and health research.