Cs-137, a radioactive isotope of caesium, is a commonly occurring fission product that is generated during the nuclear fission of U-235 and other fissionable isotopes in both nuclear reactors and weapons. Due to its long half-life of about 30 years and propensity to accumulate in sediments and marine organisms, Cs-137 is considered a major radionuclide for environmental radioactivity monitoring. In April 2021, as the Japanese government decided to discharge Fukushima contaminated water into the sea, the monitoring of marine radioactivity in South Korea has become increasingly significant. In this study, as an initial step towards establishing a standardized procedure for analyzing radioactive caesium in seawater, the radioactivity of Cs-137 was analyzed on a 2 L of seawater spiked with 10 Bq of Cs-137 standard solution supplied by KRISS. The seawater was collected from Im-nang Beach, situated at a distance of approximately 2 kilometers from DIRAMS. The radioactivity of Cs-137 in seawater was determined according to the improved AMP procedure presented by M.Aoyama in 2000. The seawater was pretreated using Ammonium Phosphomolybdate (AMP) coprecipitation, which has a high selectivity for caesium (Kd = ~5500), and the activity of Cs-137 was determined by gammaspectroscopy and subsequently corrected via the weight yield. The weight yield of the dried AMP/Cs compound was more than 93%. For the gamma-spectroscopy analysis, the AMP/Cs compound was dissolved in a cylindrical U8 beaker with NaOH to ensure that its shape and volume were consistent with the CRM (KRISS, 221U890-1) used to calibrate the detector. The dissolved compound was then positioned directly onto the detector housing and subjected to a measurement duration of 80,000 seconds utilizing a p-type HPGe (Ortec, GEM60) with a relative efficiency of 54%. The activity of Cs-137 was determined to be 10.81 Bq, confirming the reproducibility of the AMP coprecipitation and weight yield methods. The present experiment was carried out using a 2 L sample, but a large volume of seawater would be required to achieve a sufficient minimum detectable activity (MDA) for Cs-137 in natural seawater. Thus, a standardized procedure for analysis of radioactive caesium in natural seawater will be established through the analysis of a large volume of seawater in future studies.