RNA remodeling proteins, including RNA helicases and chaperones, act to remodel RNA structures and/or protein-RNA interactions, and are required for all processes involving RNAs. Although many viruses encode RNA helicases and chaperones, their in vitro activities and their roles in infected cells largely remain elusive. Noroviruses are a diverse group of positive-stranded RNA viruses in the family Caliciviridae, and constitute a significant and potentially fatal threat to human health. Here we report that protein NS3 encoded by human norovirus has both ATP-dependent RNA helicase activity that unwinds RNA helices and ATP-independent RNA chaperoning activity that can remodel structured RNAs and facilitate strand-annealing. Moreover, NS3 can facilitate viral RNA synthesis in vitro by norovirus polymerase. NS3 may therefore play an important role in norovirus RNA replication. Lastly, we demonstrate that the RNA remodeling activity of NS3 is inhibited by guanidine hydrochloride, an FDA-approved compound and, more importantly, that it reduces the replication of norovirus replicon in cultured human cells. Altogether, these findings are the first to demonstrate the presence of RNA remodeling activities encoded by Caliciviridae, and highlight the functional significance of NS3 in noroviral life cycle.