Abstract: Reinjection into landfill is a commonly adopted strategy for managing landfill leachate membrane concentrate (LLMC). However, its impacts on the hydraulic performance of geosynthetic clay liners (GCLs) remain unclear. In the present study, a series of hydraulic conductivity tests were conducted to investigate the behavior of the hydraulic conductivity of GCLs permeated with two different LLMCs (i.e., typical and strong LLMC). Free swell index tests and scanning electron microscope tests were carried out to reveal the possible mechanism responsible for the changes in hydraulic conductivity. The test results show that permeation of LLMCs leads to a substantial increase in hydraulic conductivity of GCLs, ranging from approximately 1.96 to 54.72 times. For a given GCL, the impact of strong LLMC is more pronounced than that of general LLMC. The sodium-activated bentonite GCL is more sensitive compared to the natural sodium bentonite GCLs when permeated with the same LLMC. The increase in the hydraulic conductivity could be mainly ascribed to the contraction of the electric double layer of the bentonite particles. All the results show that the LLMC will decrease the hydraulic performance of GCL to varying degrees. Thus, the recirculation of LLMC should be limited in the practical field. This study provides a theoretical foundation for the assessment of hydraulic performance of GCLs used in municipal solid waste landfill.