2013 SY99, also known by its OSSOS survey designation uo3L91, is a trans-Neptunian object discovered on September 29, 2013 by the Outer Solar System Origins Survey using the Canada–France–Hawaii Telescope at Mauna Kea Observatory. This object orbits the Sun between 50 and 1,300 AU (7.5 and 190 billion km), and has a barycentric orbital period of nearly 20,000 years. It has the second largest semi-major axis yet detected for an orbit with a perihelion beyond the zone of strong influence of Neptune (q > 38), second only to 541132 Leleākūhonua, but exceeding the semi-major axes of Sedna, 2012 VP113 and 2010 GB174. 2013 SY99 has one of highest perihelia of any known extreme trans-Neptunian object, behind sednoids including Sedna (76 AU), 2012 VP113 (80 AU), and Leleākūhonua (65 AU).