Li-S batteries are a promising next-generation battery technology but are confronted with a series of fundamental challenges, in particular the notorious shuttle effect of soluble polysulfide intermediates. Current research efforts are mostly focused on designing proper host materials for the entrapment and immobilization of polysulfides. Herein, we demonstrate that electrocatalysis may play an unexpected role in Li-S batteries. Nanosized tungsten carbide particles dispersed on the carbonaceous support are prepared from the pyrolysis of phosphotungstic acid-functionalized metal-organic frameworks. Both experimental measurements and theoretical calculations demonstrate that they not only have strong affinity toward polysulfide intermediates, but also significantly accelerate the reduction of low-order polysulfides that is otherwise kinetically challenged. Using these supported tungsten carbide nanoparticles as the cathode catalyst, our Li-S batteries achieve large capacity, excellent cycling stability and impressive rate capability.