Significance We reveal the existence of a topological phase transition (Kosterlitz–Thouless type) in grain boundaries (GBs)—important internal surfaces in crystalline materials. GB dynamics are controlled by the formation/migration of line defects (disconnections) with dislocation and step character. Below the GB KT transition, disconnections of opposite signs are bound as pairs, while above it they unbind and proliferate. We demonstrate that GB KT transitions provide a fundamental understanding of many GB properties, the temporal evolution of microstructure, and how it deforms/degrades under stress.