A method is developed to measure ¹³⁷Cs in seawater. It entails the use of silica gel impregnated with hexacyanoferrates as an ion-exchanger to concentrate ¹³⁷Cs from large volumes of seawater, followed by gamma-ray spectrometry with a high efficiency HPGe detector. Both laboratory experiments and field operation resulted in satisfactory recovery of ¹³⁷Cs. Compared with the traditional pre-concentration method using AMP as the scavenger, this method is more rapid, more cost-effective, and can be more easily performed aboard ships.