Large amounts of oyster shells are often dumped in natural water and landfills, causing pollution and health/ sanitation issues. It is highly desirable to convert oyster shell wastes into high-value-added products. In this study, an oyster shell waste-based adsorbent, polyethylenimine-crosslinked synthesized calcium silicate hydrate (PEI/S-CSH), was developed through a two-step processing route consisting of CSH synthesis and PEI crosslinking. The prepared adsorbent was characterized using FT-IR, XRD, BET, FE-SEM, and Zeta potential analyzer, and the results showed that the PEI/S-CSH was successfully prepared. In addition, the adsorption performance of PEI/S-CSH was investigated for a reactive dye, Reactive Yellow 2 (RY2), and adsorption experiments were conducted for variables such as pH value, initial concentration, and time. The PEI/S-CSH removed more than 90% of the initial RY2 concentration in the pH range of 2-7, and was almost unaffected in the NaCl concentration range of 0.01-0.1M. The maximum RY2 uptake of PEI/SCSH by the Langmuir model was estimated to be 235.0 and 156.0mg/g at pH 2 and 7, respectively. The adsorption equilibrium was affected by the pH change and equilibrium was reached within 10min at pH 2 and 30min at pH 7. The reusability of PEI/S-CSH was investigated through repeated adsorption/desorption evaluation for a total of five times. As a result, PEI/S-CSH showed good adsorption/desorption performance for RY2 up to five times. Therefore, PEI/S-CSH can be considered as an adsorbent with high potential for removing reactive dyes.