Eukaryotic mRNA turnover is among most critical mechanisms that affect mRNA abundance and are regulated by mRNA-binding proteins and the cytoplasmic exosome. A functional protein, guanosine-triphosphate-binding protein 1 (GTPBP1), which associates with both the exosome and target mRNAs, was identified. The overexpression of GTPBP1 accelerated the target mRNA decay, whereas the reduction of the GTPBP1 expression with RNA interference stabilized the target mRNA. GTPBP1 has a putative guanosine-triphosphate (GTP)-binding domain, which is found in members of the G-protein family and Ski7p, a well-known core factor of the exosome-mediated mRNA turnover pathway in yeast. Analyses of protein interactions and mRNA decay demonstrated that GTPBP1 modulates mRNA degradation via GTP-binding-dependent target loading. Moreover, GTPBP1-knockout models displayed multiple mRNA decay defects, including elevated nocturnal levels of Aanat mRNA in pineal glands, and retarded degradation of TNF-α mRNA in lipopolysaccharide-treated splenocytes. The results of this study suggest that GTPBP1 is a regulator and adaptor of the exosome-mediated mRNA turnover pathway.