Conventional techniques for surface waters sampling such as spot or automated sampling are generally time-consuming, do not take into account temporal variability and could induce samples contamination. The Diffusive Gradient in Thin films (DGT) technique is an alternative sampling method which has been widely used to determine “labile” metal concentrations in surface waters. However, classical DGT based on a polyacrylamide diffusive gel and a Chelex resin do not allow trapping Hg, which is one of the priority substances of the Water Framework Directive. Consequently, the development of a home-made passive sampler combined with gas chromatography-inductively coupled plasma-mass spectrometry (GC-ICP-MS) is necessary to measure low levels (ng/L) of inorganic mercury Hg(II) and methylmercury MeHg in freshwaters. We developed, in clean conditions, a home-made Hg-specific passive sampler based on an agarose diffusive gel and a 3-mercaptopropyl functionalized silica resin. It was tested in laboratory and deployed in situ for the quantification of Hg(II) and MeHg in freshwaters. We obtained much lower level of blank (0.03 ng Hg) in home-made DGT compared to commercial DGT (0.80 ng Hg) resulting in quantification limit improvement. The diffusion coefficients calculated from kinetic experiments in the home-made agarose gel were 3.74.10-6 and 2.75.10-6 cm²/s for Hg(II) and MeHg, respectively. The quantification of Hg species trapped in the home-made resin was achieved by isotope dilution, extracted with acidic thiourea solution and derivatized for a measurement by GC-ICP-MS. We demonstrated the usefulness of our tool through its deployment in weakly (0.2-0.3 ng/L) and strongly contaminated (10-15 ng/L) rivers.