This study provides the first comprehensive baseline of biomass and productivity for mangrove forests in the understudied “polyhaline zone” of the Sundarbans Reserve Forest (SRF), Bangladesh, with significant implications for global carbon accounting and climate resilience strategies. Over a one-year period (September 2022 to September 2023), we quantified changes in stand structure, above- and below-ground biomass, and litterfall in a 700 m² sample plot, establishing critical reference values for future monitoring and conservation efforts. Excoecaria agallocha dominated numerically, though Avicennia officinalis contributed most to biomass stocks despite lower stem density, revealing important species-specific carbon storage efficiencies. The total above-ground biomass (AGB) and below-ground biomass (BGB) were 414 Mg ha⁻¹ and 365.5 Mg ha⁻¹, respectively, with an unusually high below-ground allocation (47% of total biomass) compared to global averages. This distinctive carbon allocation pattern suggests adaptive strategies to high-salinity environments and represents a significant, stable carbon reservoir. The mean annual litterfall was 5.15 Mg ha⁻¹ yr⁻¹, with reproductive components contributing 58.4% of litterfall biomass, indicating high reproductive investment under salinity stress. Net primary productivity (NPP) was 29.9 Mg ha⁻¹ yr⁻¹, with above-ground NPP contributing 66.4%, demonstrating substantial carbon sequestration capacity even under high salinity conditions. The above-to-below-ground biomass ratio averaged 1.3, significantly lower than terrestrial forests, highlighting the distinctive carbon storage mechanisms of mangroves in high-salinity environments. This baseline study advances our understanding of how mangrove ecosystems function as carbon sinks and provides reference data for climate change mitigation strategies, Blue-Carbon Initiatives, and resilience-focused conservation management in the similar ecosystems globally.