A custom radiation monitoring system was developed by Oregon State University at the request of the Woods Hole Oceanographic Institute to measure radioactive cesium contaminants in the ocean waters near Fukushima Dai-ichi Nuclear Power Plant. The system was to be used on board the R/V Ka’imikai-O-Kanaloa during a 15 d research cruise to provide real-time approximations of radionuclide concentration and alert researchers to the possible occurrence of highly elevated radionuclide concentrations. A NaI(Tl) scintillation detector was coupled to a custom-built compact digital spectroscopy system and suspended within a sealed tank of continuously flowing seawater. A series of counts were acquired within an energy region corresponding to the main photopeak of 137Cs. The system was calibrated using known quantities of radioactive 134Cs and 137Cs in a ratio equating to that present at the reactors’ ocean outlet. The response between net count rate and concentration of 137Cs was then used to generate temporal and geographic plots of 137Cs concentration throughout the research cruise in Japanese coastal waters. The concentration of 137Cs was low but detectable, reaching a peak of 3.8 ± 0.2 Bq/L.