Significance The posttranslational methylation of arginine is a widespread epigenetic modification catalyzed by the family of protein arginine methyltransferases (PRMTs). Dysregulation of PRMT expression is implicated in the pathogenesis of many diseases including human cancers. An atomic-scale understanding of the PRMT catalytic mechanism is crucial for both fundamental biological and pharmacological applications. Despite intense efforts, crystal structures of PRMT complexes with long peptides and full-length substrates have not been solved because of their inherent instability. To address this issue, we describe peptide-based transition state mimics that form stable complexes with the PRMT enzyme coactivator associated arginine methyltransferase 1 resulting in high-resolution cocrystal structures. Our findings provide an exciting approach to understanding PRMT substrate recognition and the regulation of arginine methylation.