The electrical resistivity Ï c of the underdoped cuprate superconductor YBa 2Cu 3O y was measured perpendicular to the CuO 2 planes on ultrahigh quality single crystals in magnetic fields large enough to suppress superconductivity. The incoherent insulating-like behavior of Ï c at high temperature, characteristic of all underdoped cuprates, is found to cross over to a coherent regime of metallic behavior at low temperature. This crossover coincides with the emergence of the small electron pocket detected in the Fermi surface of YBa 2Cu 3O y via quantum oscillations, the Hall and Seebeck coefficients, and with the detection of a unidirectional modulation of the charge density as seen by high-field nuclear magnetic resonance measurements. The low coherence temperature is quantitatively consistent with the small hopping integral t - inferred from the splitting of the quantum oscillation frequencies. We conclude that the Fermi-surface reconstruction in YBa 2Cu 3O y at dopings from p=0.08 to at least p=0.15, attributed to stripe order, produces a metallic state with three-dimensional coherence deep in the underdoped regime. © 2012 American Physical Society.