Biomass of mangrove seedlings serves as proxy to environmental health. Accurate assessment of mangrove seedling biomass at the site- scale is lacking, especially in the World Heritage Site of Indian Sundarbans. Sonneratia apetala (locally known as keora) is a common fresh water loving mangrove species. This study assessed the biomass of the vegetative parts (leaf, stem and root) of S. apetala seedlings from 18 sampling stations distributed in the western, central and eastern sectors of Indian Sundarbans with contrasting salinity profile. The study conducted during June 2016 revealed highest total biomass of the seedlings collected from the eastern sector (mean value = 11.313 gm m-2) followed by the western (mean value = 11.182 gm m-2) and central (mean value = 8.627 gm m-2) sectors. The role of salinity in regulating the growth and biomass of S. apetala seedlings is confirmed from the present study. Another aim of this study is to develop allometric equation considering the total biomass of the species as dependant variable (Y) and various vegetative parts (leaf, stem and root separately) as independent variables (X).