Functional foods containing antioxidant molecules can limit the prevalence of various diseases, increase immunity, slow aging, and improve physical appearance. The aim of this study was to use non-toxic to prepare extracts from the Sonneratia alba fruit, determine the phytochemical composition, and assess the biological activity via in silico and in vitro antioxidant capability. Variations in temperatures and times showed that the conditions for best phytochemicals and antioxidant activity were 100 °C and 10 min. The highest values were 23.33 mg quercetin equivalent/g (flavonoids), 1.24 mg tannic acid equivalent/g (tannins), and IC50 = 62.38 ppm (antioxidant activity). Gas chromatography displayed 7 peaks that were attributed to 21 possible compounds. Molecular docking analysis results showed that the mechanism of three of the compounds (1) Ethyl iso-allocholate, (2) Gibb-3-ene-1,10-dicarboxylic acid,2,4a,7-trihydroxy-1-methyl-8-methylene-,1,4a-lactone,10-methyl ester,(1a,2β,4aa,4bβ,10β)-, and (3) Pregn-4-ene-3,20-dione,17,21-dihydroxy-,bis(O-methyloxime) could be due to their binding via hydrogen bonding to 3 reactive oxygen species (ROS) proteins (nitric oxide synthases). Subcritical water extract of Mangrove fruit (S. alba) has potential as a functional food that has antioxidant activity in silico and in vitro.