Green tea extract (GTE) is a common component of several popular herbal supplements, including those marketed as ‘fertility-boosting’ supplements. Previous research has suggested that although GTE can be toxic to liver cells, it increases motility of sperm cells, although there is little research regarding other functional or metabolic changes in sperm. The current study aimed to elucidate the in vitro effects of GTE on ovine spermatozoa exposed to 0.1, 0.5, 1 or 5 mg/mL GTE. Flow cytometry was used to assess viability, acrosome reaction, membrane lipid disorder, mitochondrial superoxide production, intracellular reactive oxygen species (ROS) and DNA fragmentation. At 0.5, 3 and 6 h, all concentrations of GTE tested increased DNA fragmentation, whilst 0.5, 1 and 5 mg/mL promoted mitochondrial superoxide production at all time points. Intracellular ROS production also increased with 0.1, 0.5 and 5 mg/mL GTE. Despite reported antioxidant activity of GTE, the increase in both intracellular and mitochondrial ROS suggests that exposure to GTE increases oxidative stress in sperm. Additionally, the reduction in DNA integrity indicates that GTE may cause irreversible chromatin damage, whether through promoting oxidative stress or via another mechanism. Contrary to popular belief that these products enhance fertility, this research indicates that they should be used with caution due to possible detriment to sperm, and a lack of stringent safety data.