Syzygium alternifolium (SA) is a plant widely used in folk medicine. The current study was carried out to characterize the anti-Helicobacter pylori (Hp) and anti-gastric cancer activity of S. alternifolium fruit extracts. S. alternifolium total phenolics (SATP) and S. alternifolium aqueous extract (SAAE) were used to examine anti-H. pylori and H. pylori urease inhibitory activity. The anti-gastric cancer activity of these extracts was also evaluated using an MTT cell viability assay followed by cell cycle progression analysis. The in vitro results showed that both extracts have dose-dependent anti-H. pylori and anti-gastric cancer activity. SATP has significant anti-H. pylori activity against the Hp26695 and HpP12 strains with MIC50 values of 148±20 and 141±30 μg/ml, respectively, while SAAE has moderate activity compared to the antibiotic amoxicillin. SATP showed significant urease activity with an IC50 of 98±6.4 μg/ml, while SAAE showed a moderate effect compared with positive control. Moreover, SATP showed significant inhibition of human gastric adenocarcinoma cell growth by inhibiting the G2/M phase of the cell cycle with an IC50 of 77.9±0.2 μg/ml, while SAAE exhibited moderate inhibition compared with normal control. Reverse phase UV-HPLC analysis indicated the presence of the phenolic compounds cinnamic, gallic, ferulic and vanillic acid in these extracts. The results suggest that SATP and SAAE extracted from S. alternifolium have significant anti-H. pylori and anti-gastric cancer activity and offer protection against gastritis. Hence, these extracts are recommended for patients with gastrointestinal cancer.