IN BRIEF: Postovulatory oocyte aging significantly compromises oocyte quality, resulting in fertilization failure, abnormal embryonic development, and unfavorable outcomes in assisted reproductive technology (ART). Epigallocatechin-3-gallate (EGCG), a polyphenolic compound found in green tea, demonstrates the ability to effectively reduce excessive oxidative stress in oocytes and improve the quality of aged oocytes both in vitro and in vivo, offering promising potential for enhancing the success of ART and efficiency in livestock breeding. ABSTRACT: Oxidative stress-mediated postovulatory aging (POA) significantly compromises oocyte quality and impairs subsequent embryonic developmental competence, thereby reducing the efficiency of assisted reproductive technologies (ART) and livestock breeding. As the most abundant polyphenolic compound in green tea, epigallocatechin-3-gallate (EGCG) has demonstrated notable antioxidant activity. However, the mechanisms by which EGCG modulates POA remain largely unclear. This study aimed to investigate whether EGCG delays POA both in vitro and in vivo by alleviating oxidative stress. During in vitro aging, metaphase II (MII) stage mouse oocytes were treated with various concentrations of EGCG for 12 h. EGCG treatment attenuated abnormal spindle formation and restored mitochondrial function. Furthermore, EGCG reduced reactive oxygen species levels and apoptosis, thereby mitigating oxidative damage associated with postovulatory aging. Notably, these improvements led to significantly enhanced embryonic developmental potential. In the in vivo experiments, mice received daily EGCG injections for 6 consecutive days. The results demonstrated that EGCG significantly improved oocyte quality during POA and alleviated adverse pregnancy outcomes. Taken together, our findings suggest that EGCG is a promising agent for preventing postovulatory oocyte aging and provides a basis for further strategies aimed at improving the success of ART and livestock breeding.