This study was undertaken to investigate the redox potential (Eh) of sulfidic groundwater in unconsolidated sediments. The Eh was determined by long-term (several days to several weeks) continuous in situ potentiometric measurements using a platinum (Pt) electrode. The Eh values measured in two monitoring campaigns were -259 and -202 mV, respectively. Chemical analysis of groundwater showed that the redox species in the groundwater were sulfide (S²⁻) and iron, respectively. The saturation indices calculated from the chemical analysis results indicated that FeS(am) and mainly mackinawite were close to equilibrium in the analyzed waters. Comparison of the measured Eh values with those calculated using different redox couples revealed that the Eh values measured in the first monitoring campaign were nearly equal to those calculated using HS⁻/SO₄²⁻, S²⁻/SO₄²⁻, FeS(am)/SO₄²⁻, and mackinawite/SO₄²⁻ redox couples; on the other hand, the Eh values measured in the second monitoring campaign were almost consistent with those measured using the FeS₂/SO₄²⁻ redox couple. The good fit between the measured Eh values and the theoretical calculated Eh values suggests that the sulfur system is related to the Eh value of sulfidic groundwater in unconsolidated sediments.