The Large UV/Optical/Infrared Surveyor (LUVOIR) mission is one of four Decadal Survey Mission Concepts studied by NASA in preparation for the US National Academies' Astro2020 Decadal Survey. This observatory has the major goal of characterizing a wide range of exoplanets, including those that might be habitable -- or even inhabited. It would simultaneously enable a great leap forward in a broad range of astrophysics -- from the epoch of reionization, through galaxy formation and evolution, to star and planet formation. Powerful remote sensing observations of Solar System bodies will also be possible. This Final Report on the LUVOIR study presents the scientific motivations and goals of the mission concept, the engineering design, and technology development information. Please refer to the LUVOIR Final Report Appendices (separate document) for additional information.