Fragile X syndrome is the most common form of inherited mental retardation and is caused by the absence of expression of the FMR1 gene. The protein encoded by this gene, Fmrp, is an RNA-binding protein that binds a subset of mRNAs and regulates their translation, leading to normal cognitive function. Although the association with RNAs is well established, it is still unknown how Fmrp finds and assembles with its RNA cargoes and how these activities are regulated. We show here that Fmrp is post-translationally methylated, primarily on its arginine-glycine-glycine box. We identify the four arginines that are methylated and show that cellular Fmrp is monomethylated and asymmetrically dimethylated. We also show that the autosomal paralog Fxr1 and the Drosophila ortholog dFmr1 are methylated post-translationally. Recombinant protein arginine methyl transferase 1 (PRMT1) methylates Fmrp on the same arginines in vitro as in cells. In vitro methylation of Fmrp results in reduced binding to the minimal RNA sequence sc1, which encodes a stem loop G-quartet structure. Our data identify an additional mechanism, arginine methylation, for modifying Fmrp function and suggest that methylation occurs to limit or modulate RNA binding by Fmrp.