Investigating the spatial distribution of ultra-trace mercury (ng L-1) in volcanic and geothermal areas can provide insights into geological anomalies and regional volcanism. In this study, an accurate and precise method for Hg determination in geothermal water was developed using inductively coupled plasma tandem quadrupole mass spectrometry (ICP-MS/MS) and a collision/reaction cell (CRC). The experimental conditions, including the Hg isotope type and CRC gas, were optimized to eliminate severe tungsten oxide (WO) interference that occurs in conventional ICP-MS analysis of Hg. When O2 was used as the CRC gas in the MS/MS mode, the limit of quantitation (LOQ) for Hg was 0.5 ng L-1. The proposed method was successfully applied to the direct determination of Hg in a groundwater-certified reference material (ERM-CA615) and five geothermal water samples, two of which had a high W content. The satisfactory results indicate the significant potential of the proposed for determining ultra-trace levels of Hg in geological samples.