Mangrove leaves of Rhizophora mucronata contain various secondary metabolites, such as tannins, carotenoids, phenols, chlorophyll, and alkaloids, which have potential for development as tisane. Lactobacillus plantarum SK(5) was isolated from the fermented fish product bekasam. This study aimed to determine the effect of biotransformation duration of mangrove leaves by L. plantarum SK(5) on the chemical and microbiological characteristics of antioxidant-rich tisane. Biotransformation was carried out at 37 °C for 4 days. Every day, total lactic acid bacteria (LAB) counts, pH measurements, total titratable acidity (TAT), total phenols, tannins, and antioxidant activity were determined. The experimental design used was a completely randomized design with one factor, namely fermentation duration (0, 1, 2, 3, and 4 days) and three replications. Differences in fermentation duration had a significant effect (p<0.05) on pH, TAT, tannin content, and antioxidant activity, but did not significantly affect total LAB and total phenols. The best fermentation duration was on the 2nd day, when the tisane had the following characteristics: LAB count 1.7 log CFU/mL, pH value 4.9, TAT 2.4%, total phenols 4.32 mg/g, total tannins 0.05%, and an IC50 value of 69.6 mg/mL, which falls into the strong antioxidant category. Biotransformation of R. mucronata mangrove leaves by L. plantarum SK(5) has potential as a safe tisane containing bioactive compounds with strong antioxidant activity. These findings may serve as a basis for further development of functional food products based on biotransformation through fermentation from local coastal resources of Indonesia.