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.IMPORTANCE Noroviruses are a diverse group of positive-stranded RNA viruses, which annually cause hundreds of millions of human infections and over 200,000 deaths worldwide. For RNA viruses, cellular or virus-encoded RNA helicases and/or chaperones have long been considered to play pivotal roles in viral life cycles. However, neither RNA helicase nor chaperoning activity has been demonstrated to associate with any norovirus-encoded proteins, and it is also unknown whether norovirus replication requires the participation of any viral or cellular RNA helicases/chaperones. We found that a norovirus protein NS3 not only has ATP-dependent helicase activity, but also acts as an ATP-independent RNA chaperone. And NS3 can facilitate in vitro viral RNA synthesis, suggesting the important role of NS3 in norovirus replication. Moreover, NS3 activities can be inhibited by an FDA-approved compound, which also suppresses norovirus replicon replication in human cells, raising the possibility that NS3 could be a target for anti-noroviral drug development.