Pulsars are difficult to discover via gamma-ray pulsations because typically only one photon is observed per ~${10}^8$ rotations. We report the first gamma-ray pulsar discovered with an extension of a recently proposed algorithm, which uses efficient tools for identifying short vectors in high-dimensional lattices instead of exhaustively searching for periodic signals. PSR J1622−7204, an isolated pulsar spinning at 261 Hz, was found in 15 years of Fermi-LAT data using about ten CPU hours. In contrast, the previous state-of-the-art (a semi-coherent search) failed after ~600 CPU years. This new approach may enable the discovery of many new gamma-ray pulsars, including in binary systems, a previously intractable problem without detailed prior knowledge of the orbital parameters.