Metamorphic veins record the fracture-controlled flow of fluids throughout the oceanic and continental crust. I show that the spatial distributions of veins from three diverse metamorphic settings are fractal and self-similar. Vein densities were measured by counting the number of macroscopic veins intersected along linear transects. The localities included wol-lastonite-quartz veins in marbles, for which the fractal dimension D is 0.46, actinolite-chlorite veins in hydrothermally altered oceanic diabases (D = 0.81), and epidote-quartz veins in contact-metamorphosed basalts (D = 0.25-0.63). The fractal clustering of veins provides a geometric framework for understanding spatial and temporal patterns of fluid flow and mineral reaction during metamorphism.