<p>The sedimentary geochemistry of St. Martin’s Island is important to determine the origin of the source rock, paleo weathering, tectonic setting, sediment recycling, maturity, sorting, redox condition, and paleo salinity of the sediments. Major oxides, trace elements, and rare earth elements (REEs) obtained from the INAA technique are presented by analyzing the sediment samples collected from the shoreline of St. Martin’s Island, Bangladesh. The elemental ratios, comparison with average upper continental crust (UCC), binary diagrams (Th/Sc vs. Sc, La/Th vs. Hf, Th/Co vs. La/Sc), and chondrite normalized REE patterns exhibit substantial LREE enrichment, relatively flat HREE fractionation, considerable negative Eu anomalies (average: 0.72), indicates the derivation from a source dominated by felsic rock, with contribution from intermediate source and mafic component. Sediments from St. Martin’s Island exhibit the deposition of sediments in transitional environments of active and passive continental margin settings. Weathering indices value of CIA, PIA, CIW, CIX, and K₂O/Rb ratio show moderate chemical weathering, indicating that the sediments are chemically mature. Sedimentary redox indicative proxies, such as U/Th, V/Cr, and V/Sc, show an oxic depositional environment during sediment deposition. The intermediate CIA and other weathering index values of the St. Martin’s sediments show that the area had semiarid and humid climatic conditions throughout the deposition. The Rb/K ratio of the St. Martin’s sediments suggests that the development and deposition of the sedimentary sequence of St. Martin’s Island mainly occurred in a brackish water environment during the geological past.</p>