Herein, NbN nanocrystals immobilized on N‐doped carbon nanosheets to functionalize a polypropylene (PP) membrane (NbN@NC/PP) with a thin coating of only 4 µm are designed and synthesized. The functional modifier layer allows for sulfur‐involved transformations and also lithium plating behaviors. On the one hand, the sulfur cell with NbN@NC/PP separator exhibits excellent cycling stability and rate capacity. The good electrochemical performance partially results from the strong chemical interactions between NbN and lithium polysulfides via the formation of NbS and NLi bonds, which is proven by the first‐principles calculations and X‐ray photoelectron spectroscopy analyses. The formation of tiny nanoscrystals (<2 nm) and clusters tends to maximize the surface of NbN to interact with polysulfides and enable the effective catalysis over the sulfur‐involved reactions. The higher exchange current density and Li⁺ diffusion coefficient of NbN@NC/PP cells experimentally verify that the introduction of NbN indeed catalytically accelerates the reaction kinetics. On the other hand, the performance of Li//Li symmetric cells demonstrates that the NbN@NC modifier layer can well induce homogeneous lithium deposition. This work confirms the application potential of NbN in lithium–sulfur batteries and encourages the exploration of prospective nitrides to engineer high performance next‐generation batteries.