The 24-year-old was named by coach Joachim Loew in Germany's 27-man FIFA World Cup squad which will be reduced to 23 just before the competition starts on 11 June.
Jansen is at the squad's training camp in Sicily and is making good progress on a rehabilitation programme having picked up a foot injury in March. "Marcell has made fantastic progress. If he continues like this, he will be fit in time for the tournament," said Germany's fitness coach Shad Forsythe.
Jansen has not played since 21 March. He has won 30 caps for Germany and was in the squad for the 2006 FIFA World Cup and UEFA EURO 2008. He has not played for the national side since August 2009, but has put in consistent performances for Hamburg in the Bundesliga.