Sebastian Ernst (born 11 October 1984 in Gelsenkirchen) is a German sprinter who specializes in the 200 metres. His personal best time is 20.36 seconds, achieved during the heats at the 2004 Summer Olympics. This ranks him third among German 200m sprinters, only behind Tobias Unger and Frank Emmelmann. Ernst has represented the sports team TV Wattenscheid 01 and Schalke 04 domestically.
As a junior, he finished sixth at the 2002 World Junior Championships. The following year he took the 200 m title at the 2003 European Athletics Junior Championships, as well as winning a silver medal in the relay. He made his senior international debut the following year at the 2004 IAAF World Indoor Championships, where he narrowly missed a spot in the 200 m finals. An Olympic debut came later that season for the nineteen-year-old and, after running a personal best of 20.36 seconds in the heats, he was eliminated in the semi-finals as the fastest non-qualifier.
His 2005 season was highlighted by a 200 m and 4 × 100 metres relay silver medal double at the 2005 European Athletics U23 Championships. He also made his first appearance at the World Championships in Athletics that year, but was eliminated in the quarter-finals. He began the following year by helping Germany to third in the Swedish medley relay at the 2006 European Athletics Indoor Cup. He claimed his first national title with a win at the German Athletics Championships and represented his country in the event at the 2006 European Athletics Championships, reaching the semi-finals. In the relay event he finished fifth with teammates Alexander Kosenkow, Marius Broening and Ronny Ostwald.