Rapid　industrialization　and　urbanization　have　significantly　increased　wastewater　discharge　into　natural　water　bodies,　thereby　emphasizing　the　critical　importance　of　precise　long-term　pollutant　monitoring　at　urban　sewage　outfalls　to　protect　aquatic　ecosystems.　Free　chlorine　(FC),　widely　used　in　disinfection,　poses　significant　ecological　risks　when　overdischarged.　However,　achieving　reliable　long-term　FC　monitoring　at　discharge　points　remains　a　substantial　challenge.　In　this　study,　we　developed　a　continuously　responsive　surface-enhanced　Raman　scattering　(SERS)　sensor　for　long-term　on-site　FC　monitoring.　The　sensor　integrated　a　flexible　SERS　probe　functionalized　with　4-aminothiophenol　(4-ATP)　for　selective　FC　detection　and　a　compact　floating　roller　device　for　continuous　signal　acquisition.　The　sensor　exhibited　a　low　detection　limit　of　4.16　ppb　and　accurately　quantified　FC　concentrations　in　real-world　water　samples,　showcasing　high　sensitivity　and　reliability.　Long-term　deployment　at　sewage　treatment　plant　(STP)　outfalls　further　validated　its　robust　monitoring　capabilities　under　actual　environmental　conditions.　This　innovative　SERS　sensor　offers　a　practical　and　accurate　solution　for　long-term　on-site　FC　monitoring,　highlighting　its　potential　as　a　scalable　approach　for　environmental　monitoring　applications.　©　2025　American　Chemical　Society