Sluggish sulfur redox kinetics remain a critical bottleneck in the advancement of high-performance lithium-sulfur batteries (LSBs). Single-atom catalysts (SACs) offer a promising solution to this limitation, particularly when their coordination structures are carefully engineered. Here, we develop a chromium-based SAC featuring a unique undercoordinated CrN₃ configuration to boost sulfur electrochemistry. Compared with conventional CrN₄, the CrN₃ motif lowers 3d orbital occupancy and meanwhile activates the in-plane hybridizations with S 3p orbitals upon interaction with polysulfides, contributing to moderate adsorption strength and reduced energy barriers for bidirectional sulfur conversions. Additionally, the integration of the two-dimensional (2D) porous framework ensures abundant electrochemically active surfaces and efficiently exposed active sites. As a result, CrN₃-based cells demonstrate fast and durable sulfur redox reactions, enabling an ultralow capacity decay of 0.0075% per cycle over 1000 cycles and a high-rate capability of 651.9 mAh·g⁻¹ at 5 C. The CrN₃ catalyst retains robust catalytic efficiency under demanding conditions, delivering a high areal capacity of 5.53 mAh·cm⁻² at high sulfur loading and lean electrolyte. This work establishes a compelling paradigm of SAC coordination engineering for designing advanced sulfur electrocatalysts for next-generation LSBs.