A rapid and efficient analytical method was developed for the determination of plutonium in large-volume seawater to overcome the limitations of conventional coprecipitation techniques, which typically require an analysis time of 1–2 d and involve multiple labor-intensive steps. The proposed method integrates microwave-assisted acid digestion, CaF2/LaF3 coprecipitation, and TEVA extraction chromatography with actinide-selective resin (AN-resin) to enable high recovery and significantly reduce the analysis time. Additionally, a key challenge was overcome through method optimization using seawater pre-equilibrated with 239Pu and 242Pu as an internal tracer, namely the efficient and rapid separation of Pu from the AN-resin eluate while maintaining its oxidation state. Validation using 20 L seawater samples spiked with 239Pu achieved recoveries of 86.2 ± 4.4% and 90.5 ± 4.5% for 242Pu, with an analytical accuracy of 96.7% and precision (relative standard deviation) of 3.1%. The total pretreatment time, including a 2 h preconcentration step, was reduced to ∼7 h, while achieving detection limits of 0.03–0.05 μBq/kg. Interlaboratory comparisons confirmed the robustness of the developed method, while field application to Korean coastal waters yielded 239+240Pu activities and isotopic ratios consistent with regional monitoring data. This study presents a high-throughput and reliable alternative to conventional methods, offering a versatile platform for emergency response and environmental monitoring of plutonium and other actinides in marine systems.