Synbiotic yogurt products can be developed using mangrove apple extract inulin as a prebiotic in combination with Lactobacillus plantarum to form a probiotic. Therefore, this study aimed to determine the optimal concentrations of inulin extracted from mangrove apple for the viability of L. plantarum as a synbiotic in vitro and its antibacterial activity against Staphylococcus aureus. The samples were divided into 5 groups and 4 replicates consisting of different concentrations of inulin extracted from mangrove apple (IEMA) at 0, 3, 6, 9, and 12% w/v, which was combined with L. plantarum as a synbiotic. The result showed that the addition of IEMA with different concentrations had a significant effect (p<0.05) on the viability of L. plantarum in gastric juice and bile salt after 5 h of exposure. The total bacteria significantly decreased (p<0.05) after 4 weeks of storage period. The antibacterial activity of IEMA at concentrations of 9% and 12% was higher than other treatments as demonstrated by p<0.05. IEMA increased the viability with the concentration of 9% being the most effective (p<0.05) on acid, bile salt, resistance, and storage periods. Based on the results, IEMA can also inhibit the growth of S. aureus.