An automated analytical method implemented in a novel dual-column tandem sequential injection (SI) system was developed for simultaneous determination of U-236, Np-237, Pu-239, and Pu-240 in seawater samples. A combination of TEVA and UTEVA extraction chromatography was exploited to separate and purify target analytes, whereupon plutonium and neptunium were simultaneously isolated and purified on TEVA, while uranium was collected on UTEVA. The separation behavior of U, Np, and Pu on TEVA UTEVA columns was investigated in detail in order to achieve high chemical yields and complete purification for the radionuclides of interest. Pu-242 was used as a chemical yield tracer for both plutonium and neptunium. U-238 was quantified in the sample before the separation for deducing the U-236 concentration from the measured U-236/U-238 atomic ratio in the separated uranium target using accelerator mass spectrometry. Plutonium isotopes and Np-237 were measured using inductively coupled plasma mass spectrometry after separation. The analytical results indicate that the developed method is robust and efficient, providing satisfactory chemical yields (70-100%) of target analytes and relatively short analytical time (8 h/sample).