Mangrove forests serve as significant carbon sinks and play a critical role for climate change adaptation and mitigation; however, increasing salinity in coastal areas exerts detrimental effects on sediment carbon storage and nutrient cycling. Here, we investigated spatial variations in total organic carbon (TOC), total nitrogen (TN), sulfide, and sediment properties, including salinity, pH, and grain sizes, along with their interrelations in different salinity gradients (from low saline to high saline areas) in the Sundarbans natural mangrove forest in Bangladesh. In this study, sediment samples were taken from 0-15 cm, 15-30 cm, and 30-50 cm depths at six different sampling locations from low-saline, mid-saline, and high-saline areas. Results revealed that sediment pH, salinity, clay, and silt concentrations varied significantly both among the sampling areas and depth. Therefore, TOC and sulfide concentrations were significantly lowered in high-saline area sediment, whereas TN showed a similar distribution pattern in different salinity gradient areas. Sulfide showed a strong salinity-dependent decline across the gradient (low to high salinity), with its relationship to organic carbon shifting from pH-driven at low salinity (sulfide–pH r = -0.89) to TOC-driven at higher salinity (sulfide–TOC r = 0.73), indicating that increasing salinity progressively decouples sulfide production from acidification and recouples it to organic matter availability. Principal component analysis (PCA) revealed that the first two principal components explained 67.0% of the total environmental variation. PC1 (43.3%) was positively associated with total organic carbon (TOC), sulfide, and silt, but negatively associated with salinity, indicating that carbon accumulation was favored under low-salinity, sulfide-rich sediment conditions. PC2 (23.7%) was primarily influenced by TN and pH, with a negative contribution from clay, highlighting the secondary role of nutrient availability and sediment texture in shaping sediment biogeochemical characteristics. This research highlights complex interactions between salinity gradient, TOC, and sulfide, which reveal that sulfide could be the biogeochemical indicator for carbon storage and salinity in mangrove sediments.