To realize the practical application of lithium–sulfur (Li–S) batteries, bifunctional binders featured with the capability of trapping soluble polysulfide species besides the strong binding property are highly desired. Herein, we demonstrate the strong potential of a commercially available, environmentally friendly, water-soluble linear polyethylenimine (PEI) as a superior bifunctional binder for high-sulfur-loading cathodes in Li–S batteries. Our investigation shows the significantly improved cathode performance with enhanced sulfur utilization (i.e., higher capacity), reduced capacity decay, and longer cycling life upon the use of PEI as the binder, relative to the traditional polyvinylidene fluoride (PVDF) binder. This arises from the significantly stronger binding strength and valuable polysulfide trapping ability of the linear PEI binder. In particular, its superior polysulfide adsorption capability has been evidenced experimentally with both ex situ and in situ studies, as well as through theoretical density functional theory (DFT) calculations. At a sulfur loading of 2.4 mg cm^–2, the capacity decay rate of the cathode with the linear PEI binder is reduced to as low as 0.042% per cycle over 500 cycles at 2 C. In addition, it also enables the fabrication of high-sulfur-loading cathodes (as high as 6.5 mg cm^–2) with high areal capacity (ca. 4.5 mAh cm^–2) and high cycling stability. With its superior performance, linear PEI is promising for fabricating high-sulfur-loading cathodes of significantly lowered capacity decay for practical applications.