The survival of motor neurons protein (SMN) is part of a large complex that contains six other proteins, Gemins2-7. The SMN complex assembles the heptameric Sm protein core on small nuclear RNAs (snRNAs) and plays a critical role in the biogenesis of snRNPs, the major and essential components of mRNA splicing in eukaryotes. For its function, the SMN complex binds Sm proteins and snRNAs, which it distinguishes from other RNAs by specific features they contain. We show here that Gemin5, a 170 kDa WD-repeat protein, is the snRNA binding protein of the SMN complex. Gemin5 binds directly and specifically to the unique features, including the Sm site, of snRNAs. Reduction of Gemin5 results in reduced capacity of the SMN complex to bind snRNAs and to assemble Sm cores. Gemin5 therefore functions as the factor that allows the SMN complex to distinguish snRNAs from other cellular RNAs for snRNP biogenesis.