Based on detailed x-ray diffraction and transmission electron microscopy we have found body-centered-cubic ͑bcc͒ Ni upon room-temperature rolling of nanocrystalline ͑nc͒ face-centered-cubic ͑fcc͒ Ni. The bcc phase forms via the Kurdjumov-Sachs ͑KS͒ martensitic transformation mechanism when the von Mises equivalent strain exceeds ϳ0.3, much higher than accessible in tensile testing. The fcc and bcc phases keep either the KS or the NishiyamaWasserman orientation relationship. Our results provide insights into the deformation physics in nc Ni, namely, the fcc-to-bcc phase transformation can also accommodate plasticity at large plastic strains.