Small nuclear ribonucleoproteins (snRNPs) are core components of the spliceosome. The U1, U2, U4, and U5 snRNPs each contain a common set of seven Sm proteins. Three of these Sm proteins are posttranslationally modified to contain symmetric dimethylarginine (sDMA) residues within their C-terminal tails. However, the precise function of this modification in the snRNP biogenesis pathway is unclear. Several lines of evidence suggest that the methyltransferase protein arginine methyltransferase 5 (PRMT5) is responsible for sDMA modification of Sm proteins. We found that in human cells, PRMT5 and a newly discovered type II methyltransferase, PRMT7, are each required for Sm protein sDMA modification. Furthermore, we show that the two enzymes function nonredundantly in Sm protein methylation. Lastly, we provide in vivo evidence demonstrating that Sm protein sDMA modification is required for snRNP biogenesis in human cells.