Cochin is known for its deep deposits of soft marine clays, characterized by low density, poor shear strength, high compressibility, and an expansive nature. These properties make the soil challenging for construction and development, necessitating the use of soil stabilization techniques. Sisal fiber, a natural material, enhances the soil’s strength and durability. Seashells, often considered as waste, can be crushed and used as a stabilizing agent, improving soil’s cohesion and reducing its susceptibility to erosion. In this study, Cochin marine soil is stabilized by adding sisal fibers and seashell powder in varying proportions. By blending the soil with various stabilizer concentrations, the geotechnical properties of stabilised Cochin marine soil are examined. A comparative analysis of the compaction characteristics, free swell characteristics, unconfined compressive strength (UCS), and California bearing ratio (CBR) of both stabilized and unstabilized soil is carried out in this study. The optimum dosage of seashell powder and sisal fiber dosages are observed to be 7% and 0.6%, respectively. The results indicate that addition of optimum amount of sisal fibers improved the UCS of the soil, increasing it from 2.38 kPa to 4.62 kPa. An increase of CBR from 4.03 to 17.98 is observed when the soil is stabilized with sisal fibers and seashell powder. At addition of sisal fibers and seashell powder, a decrease in free swell index from 50% to 30% is observed. The addition of sisal fibers and seashell powder not only strengthens the soil but also promotes environmental sustainability by utilizing renewable and recyclable materials.