In the past few years, atomically thin layered semiconductor materials have attracted numerous interest because of the potential of using those materials to establish liquid–semiconductor sensor systems towards next-generation biological/chemical sensing applications. However, the information regarding the electrical characteristics of 2D materials in aqueous environments for advancing the development of such emerging sensor systems is still unclear. In this report, we have synthesized the bilayer molybdenum disulfide (MoS₂) nanomaterials by the chemical vapor deposition method. The layer number of the as-grown MoS₂ materials were characterized by using both the Raman spectra and the atomic force microscope (AFM). The selected bilayer MoS₂ materials were transferred to a silicon substrate covered with 100 nm-thick SiO₂ dielectric layer. The source/drain electrode patterns of the back-gate FETs were defined using an electron beam lithography, followed by the deposition process of the gold thin layer. Importantly, the source/drain electrodes were passivated by the PR layer for reducing the leakage current. After the FET fabrication, the electrical properties of bilayer MoS₂ FETs were measured under both the ambient condition and the different concentrations of KCl aqueous electrolyte solutions. From the experimental result of the extracted field-effect mobility, we can evidently find that, when the ionic strength of the KCl electrolyte solution was increased from 0.1 μM to 0.1 M, the values of the ratio of the electron mobility extracted at the aqueous solution to that extracted at the air environment revealed a decreasing trend. The scattering from the ions in the aqueous electrolyte solutions may presumably give rise to the degradation of electron mobility. This report provides information regarding the dependence of the ionic strength on the mobility of atomically thin layered FETs for future sensor applications.