Type I Protein Arginine Methyltransferases (PRMTs) catalyze asymmetric dimethylation of arginine residues on numerous proteins. Type I PRMTs and their substrates have been implicated in human cancers, suggesting that inhibiting Type I PRMT activity offers a tractable approach for therapeutic intervention. This dataset contains results from immunoproteomic studies enriching for monomethylated, asymmetrically, and symmetrically dimethylated arginines in cancer cell lines treated with the Type I PRMT inhibitor, GSK3368712.