Dimethyl sulfoxide (DMSO) is an organosulfur compound with unique physicochemical properties, making it a versatile solvent and permeation enhancer in transdermal delivery systems. This chapter outlines the origin, chemistry, and characteristics of DMSO, followed by an overview of transdermal patches and microneedles, including their history, design, and classification. The role of DMSO as a chemical permeation enhancer and solvent in transdermal and microneedle-based systems is emphasized, highlighting its contribution to improved drug solubility, permeability, and bioavailability. Furthermore, DMSO’s applications in sustained-release and cryomicroneedle formulations are discussed, underscoring its potential in advanced transdermal therapeutic strategies.