ATLAS (Astrophysics Telescope for Large Area Spectroscopy) Probe is a concept for a NASA probe-class space mission that will achieve groundbreaking science in the fields of galaxy evolution, cosmology, Milky Way, and the Solar System. It is the follow-up space mission to WFIRST, boosting its scientific return by obtaining deep 1 to 4 µm slit spectroscopy for ∼ 70% of all galaxies imaged by the ∼2000 deg 2 WFIRST High Latitude Survey at z > 0.5. ATLAS spectroscopy will measure accurate and precise redshifts for ∼ 200M galaxies out to z < 7, and deliver spectra that enable a wide range of diagnostic studies of the physical properties of galaxies over most of cosmic history. ATLAS Probe and WFIRST together will produce a 3D map of the Universe with ∼Mpc resolution in redshift space over 2,000 deg 2 , the definitive data sets for studying galaxy evolution, probing dark matter, dark energy and modifications of General Relativity, and quantifying the 3D structure and stellar content of the Milky Way. ATLAS Probe science spans four broad categories: (1) Revolutionizing galaxy evolution studies by tracing the relation between galaxies and dark matter from galaxy groups to cosmic voids and filaments, from the epoch of reionization through the peak era of galaxy assembly; (2) Opening a new window into the dark Universe by weighing the dark matter filaments using 3D weak lensing with spectroscopic redshifts, and obtaining definitive measurements of dark energy and modification of General Relativity using galaxy clustering; (3) Probing the Milky Way's dust-enshrouded regions, reaching the far side of our Galaxy; and (4) Exploring the formation history of the outer Solar System by characterizing Kuiper Belt Objects. ATLAS Probe is a 1.5m telescope with a field of view (FoV) of 0.4 deg 2 , and uses Digital Micro-mirror Devices (DMDs) as slit selectors. It has a spectroscopic resolution of R = 1000, and a wavelength range of 1-4 µm. The lack of slit spectroscopy from space over a wide FoV is the obvious gap in current and planned future space missions; ATLAS fills this big gap with an unprecedented spectroscopic capability based on DMDs (with an estimated spectroscopic multiplex factor greater than 5,000). ATLAS is designed to fit within the NASA probe-class space mission cost envelope; it has a single instrument, a telescope aperture that allows for a lighter launch vehicle, and mature technology (we have identified a path for DMDs to reach Technology Readiness Level 6 within two years). ATLAS Probe will lead to transformative science over the entire range of astrophysics: from galaxy evolution to the dark Universe, from Solar System objects to the dusty regions of the Milky Way.