Coastal road pavements are highly susceptible to seawater inundation, which can accelerate deterioration. Seawater possesses aggressive and destructive properties, including high electrical conductivity, elevated dissolved oxygen levels, increased temperatures, and corrosive chloride ions, all of which can damage asphalt surfaces. This study aims to evaluate the effects of seawater immersion on continuously graded asphalt mixtures in which steel slag waste is used as a replacement for 3/8″ coarse aggregate. Asphalt mixtures were prepared with steel slag replacement levels of 0, 25, 50, 75, and 100%, and were subjected to immersion in seawater from Parangtritis Beach, Bantul Regency, Special Region of Yogyakarta, as well as in freshwater, for 24 h. Marshall properties, including density, voids filled with asphalt (VFA), stability, and flow, were then evaluated. The results show that seawater immersion generally reduced these properties compared to freshwater immersion. Mixtures containing 100% steel slag exhibited reductions of up to 2.6% in density, 3.6% in VFA, and 7.1% in stability. These findings demonstrate that while steel slag is a viable coarse aggregate replacement, its performance declines in chloride-rich environments, indicating that for coastal road applications, additional protective measures such as anti-stripping agents or surface sealants are essential to maintain long-term durability.