DMSO‐TsCl has been developed for direct methylthiolation of various electron‐rich heteroarenes (more than 40 examples) with high regioselectivity in moderate to excellent yields (up to 96 %). Especially, pyrroles, furans, and thiophenes can be efficiently mono‐methylthiolated. This practical method features scalable, metal‐free, mild conditions and is compatible with air and moisture. Several applications of methylthiolated products were demonstrated for the first time. Based on controlled experimental results, a plausible mechanism was proposed with two key intermediates involved in the mechanism being detected by HRMS. [ABSTRACT FROM AUTHOR]