We report the discovery of an X-ray periodicity of∼ 265.3 ms from a deep XMM-Newton observation of the radio-quiet γ-ray pulsar, PSR J2021+ 4026, located at the edge of the supernova remnant G78. 2+ 2.1 (γ-Cygni). The detected frequency is consistent with the γ-ray pulsation determined by the observation of the Fermi Gamma-ray Space Telescope at the same epoch. The X-ray pulse profile resembles the modulation of a hot spot on the surface of the neutron star. The phase-averaged spectral analysis also suggests that the majority of the observed X-rays have thermal origins. This is the third member in the class of radio-quiet pulsars with significant pulsations detected from both X-ray and γ-ray regimes.