Histone H3 arginine 2 (H3R2) is post-translationally modified in three different states by “writers” of the protein arginine methyltransferase (PRMT) family. H3R2 methylarginine isoforms include PRMT5-catalyzed mono- and symmetric di-methylation (me1, me2s), and PRMT6-catalyzed me1 and asymmetric dimethylation (me2a). WD-40 repeat-containing protein 5 (WDR5) is an epigenetic “reader” protein that interacts with H3R2 and is a subunit of numerous chromatin-modifying complexes, such as the Mixed Lineage Leukemia (MLL) H3 lysine 4 methyltransferase complex. Previous studies suggested that MLL recruitment to chromatin was specified by the high-affinity interaction between WDR5 and H3R2me2s. However, our prior biological data prompted the hypothesis that WDR5 may also interact with H3R2me1 to recruit MLL activity. Here, using highly accurate quantitative binding analysis combined with high-resolution crystal structures of WDR5 in complex with unmodified (me0) and me1/me2s L-Arginine amino acids and in complex with H3R2me1 peptide, we provide a rigorous biochemical study of this important biological interaction. Despite modest structural differences at the binding interface, our study supports an interaction model regulated by a binary arginine methylation switch: H3R2me2a prevents interaction with WDR5, whereas H3R2me0/me1/me2s are equally permissive.