AbstractPolymerization in the solid state is generally infeasible due to restrictions on mobility. However, in this work, the solid‐state photopolymerization of crystalline dicyclopentadiene is demonstrated via photoinitiated ring‐opening metathesis polymerization. The source of mobility in the solid state is attributed to the plastic crystal nature of dicyclopentadiene, which yields local short‐range mobility due to orientational degrees of freedom. Polymerization in the solid state enables photopatterning, volumetric additive manufacturing of free‐standing structures, and fabrication with embedded components. Solid‐state photopolymerization of dicyclopentadiene offers a new paradigm for advanced and freeform fabrication of high‐performance thermosets.