Marcell Jansen (German pronunciation: ; born 4 November 1985) is a German former footballer and current president of Hamburger SV. He was well known for his accurate crossing and pace, despite his tall stature. A versatile player, he played primarily as a full back or wing back on the left flank, but could also play as a left-winger or even as a striker.
Growing up in Mönchengladbach, West Germany, Jansen joined Borussia Mönchengladbach from local SV Mönchengladbach in 1993 and progressed through the ranks at the club's academy. It was not until 2003 when then-Mönchengladbach manager Ewald Lienen named him as an unused substitute in a Bundesliga fixture of the club in a 2–0 loss against Hannover 96 on 19 September 2003. Ironically, this turned out to be Lienen's final game in charge of Mönchengladbach and Jansen returned to the U19 side.
The start of the 2004–05 season saw Jansen playing for Borussia Mönchengladbach II in the fourth division under manager Horst Köppel. By the end of 2004, Jansen was given a first team opportunity under Dick Advocaat, who gave him his debut for the side. They lost 6–0 away against Hertha BSC on 4 December 2004. During the match, which saw Hertha BSC leading 5–0 in the second half, it went bad to worse when he gave away a penalty, resulting a successful penalty kick from Marcelinho Paraíba to score Hertha's sixth goal. Nevertheless, Advocaat continued using him in the first team following an injury to club captain Christian Ziege. On 28 January 2005, Jansen signed his first professional contract with the club, keeping him until 2008. He continued to be a first team regular at the club, playing as a left-back and because of this, he earned a call up for the Germany under-21 national team as well.