In recent years, arginine-rich basic proteins have garnered significant attention due to their essential roles in various biological processes. However, the potential of marine-derived proteins in this domain remains largely unexplored. This study presents, for the first time, the isolation and purification of a 14.3 kDa protamine (SOP) from the mature spermatogonial tissues of . Additionally, we obtained an 18.5 kDa PEGylated derivative, SOP-PEG. The physicochemical properties of both SOP and SOP-PEG were comprehensively characterized using SEM, FTIR, CD, and TGA. PEGylation markedly altered the surface morphology, secondary structure, and thermal stability of SOP. In vitro studies demonstrated that PEGylation significantly enhanced the biocompatibility of SOP, leading to improved proliferation of L-929 fibroblasts. Furthermore, both SOP and its PEGylated derivative (SOP-PEG) regulated the cell cycle, activated the PI3K-Akt signaling pathway, and modulated anti-apoptotic mechanisms, suggesting their potential to support cell survival and facilitate tissue regeneration. Notably, SOP-PEG exhibited superior bioactivity, likely attributable to its enhanced delivery efficiency conferred by PEGylation. Collectively, these findings underscore the promising applications of SOP and SOP-PEG in regenerative medicine and highlight the pivotal role of PEGylation in augmenting the bioactivity of SOP.