A diffusive gradient in thin films (DGT) technique for measuring methylmercury (MeHg) concentrations in natural waters was developed using 3-mercaptopropyl-functionalized silica gel to preconcentrate the methylmercury. The new resin was characterized and calibrated. Methylmercury is efficiently accumulated at a pH range of 3–9. Basic performance tests of the new DGT device confirmed the applicability of Fick’s first law for such DGT measurements. The diffusion coefficient of methylmercury in polyacrylamide gel was 5 × 10⁻⁶ cm² s⁻¹. Methylmercury concentrations determined by DGT deployed for different time periods in the field are statistically not different from results obtained through direct measurements. The DGT technique represents therefore an alternative in situ sampling method for methylmercury. The detection limit of the overall method is 1 pg of MeHg, which correspond to approximately 30 pg L⁻¹ of MeHg in a water sample, when deploying a typical DGT device for 24 hours. Lower MeHg concentrations are measurable using longer deployment times or thinner diffusive gel layers.