Sulfur-sulfur bonds are ubiquitous across broad classes of natural products, peptides and proteins, drug molecules, and synthetic polymers and materials. The ability to make and break these bonds in a controlled manner is critical for their many scientific and technological applications. In this study, we report the discovery of a new and unusual S-S metathesis reaction of organic trisulfides. When exposed to certain polar aprotic solvents, trisulfides were found to undergo spontaneous metathesis, with equilibrium established in seconds in some cases. No exogenous reagents, heat, light, or other stimuli were required to provoke this reaction. Understanding the scope and mechanism of this reaction enabled diverse applications of this chemistry in dynamic combinatorial library synthesis, covalent modification of complex natural products, and S-S metathesis polymerization and depolymerization as a platform for chemically recyclable plastics.1 Additional applications of these sulfur-rich polymers in optics2 and e-waste recycling3,4 will also be discussed.