Oxidative stress is one of the leading causes that contribute to the pathogenesis of chronic diseases based on the imbalance between free radicals and the antioxidant defense system. Here, Syzygium jambos (L.) Aston, a traditional herb used to treat various diseases, was evaluated using RAW264.7 macrophages with lipopolysaccharide (LPS)-stimulation to demonstrate its effects and find a new therapy for oxidative stress. This study proved that Syzygium jambos leaf extract exerted its ability to protect cells against oxidative stress via activating the nuclear factor erythroid 2-related factor 2/heme oxygenase-1 (Nrf2/HO-1) pathway to suppress reactive oxygen species (ROS) production. Moreover, the extract also attenuated the nitric oxide (NO) production and the nitric oxide synthase (iNOS) expression, suggesting an anti-inflammatory response. In addition, four active compounds, including gallic acid, quercetin, myricetin, and caffeic acid, were isolated from ethyl acetate fraction using column chromatography. The structure of compounds was analyzed based on one-dimensional nuclear magnetic resonance (1D-NMR) and compared to literature data. Their biological activity was confirmed by promoting the induction of HO-1. Hence, this study provides the initial evidence of Syzygium jambos leaves protective effects and highlights its potential as an antioxidant agent.