The paralogous proteins UPF3A and UPF3B are involved in recognizing defective mRNAs that are degraded by nonsense-mediated mRNA decay (NMD). While UPF3B has been demonstrated to support NMD, UPF3A was reported to act either an NMD activator or an NMD inhibitor. Here, we present a comprehensive functional analysis of UPF3A and UPF3B in human cells using overexpression, knockdowns, knockouts (KO) and rescue experiments. Overexpression or knockout of UPF3A did not result in detectable changes in global NMD activity or other specific transcriptome alterations . NMD activity was also virtually unchanged in UPF3B KO cells. In contrast, the co-depletion or co-knockout of UPF3A and UPF3B resulted in a marked NMD inhibition and a global upregulation of PTC-containing transcripts. In rescue experiments UPF3B was fully functional when either UPF2- or EJC binding was impaired . However, the deletion of both interaction sites or one interaction site and a central region of UPF3B impaired its NMD function. Altogether, our work identifies critical functional domains of UPF3B and establishes redundant roles of UPF3A and UPF3B during NMD.