Herein, we report that iodine-catalyzed guanylation of primary amines can be accomplished with N,N'-di-Boc-thiourea and TBHP to afford the corresponding guanidines in 40-99% yields. Oxidation of the HI byproduct by TBHP eliminates the need for an extra base to prevent the protonation of substrates and makes the reaction especially useful for both electronically and sterically deactivated primary anilines.