Protein arginine methyltransferases (PRMTs) catalyze the posttranslational methylation of arginine using S-adenosylmethionine (SAM) as a methyl-donor. The PRMT family is widely expressed and has been implicated in biological functions such as RNA splicing, transcriptional control, signal transduction, and DNA repair. Therefore, specific inhibitors of individual PRMTs have potentially significant research and therapeutic value. In particular, PRMT1 is responsible for >85% of arginine methyltransferase activity, but currently available inhibitors of PRMT1 lack specificity, efficacy, and bioavailability. To address this limitation, we developed a high-throughput screening assay for PRMT1 that utilizes a hyper-reactive cysteine within the active site, which is lacking in almost all other PRMTs. This assay, which monitors the kinetics of the fluorescence polarization signal increase upon PRMT1 labeling by a rhodamine-containing cysteine-reactive probe, successfully identified two novel inhibitors selective for PRMT1 over other SAM-dependent methyltransferases.