Climate change and urbanization have caused increased surface runoff and reduced infiltration, which together exacerbates urban flooding. The hypothesis in this study is that green engineering from renewable marine biomass is a resource-efficient approach that utilize the intrinsic properties of brown seaweed for material development and optimization to achieve functional properties with potential for stormwater management. The study focuses on understanding the structure-property relationship for material development, while making use of the entire seaweed as a resource for a 100 % yield combined with non-toxic processing. The stipes, byproducts of seaweed harvesting, were fibrillated directly without using any chemicals, at an energy consumption of 2 kWh/kg, resulting in a green paste that was subsequently diluted to set concentrations and freeze dried to obtain a porous structure. CaCl2 crosslinking of the structures was optimized to achieve a porous biomaterial with porosities between 80 % and 94 % that kept its structural integrity upon absorption via a post-crosslinking approach. The drying process was optimized to achieve dimension stability wherein a second freeze-drying cycle and solvent method resulted in structures with dimension stability, compared to the uneven shapes that was obtained for materials dried directly at room temperature. The developed porous biomaterial displayed prominent absorption capacity with deionized water and stormwater absorption capacities around 3500 % and 4000 %, respectively. The cyclic water absorption studies also confirmed that the material can withstand during multiple cycles of absorption-drying with excellent structural integrity.