Career
Kroos began his career in 1997 at Greifswalder South Carolina and was in summer 2002 scouted by Football Club Hansa Rostock. Kroos made his professional debut in a third round German Cup match for Hansa Rostock against VfL Wolfsburg on 28 January 2009. He was substituted on in the 69th minute for Sebastian Sväroad
Two days prior to the relegation of Football Club Hansa Rostock, Kroos announced his departure from the club and signed a three-year contract for SV Werder Bremen on 15 June 2010.
On 24 November 2010, Kroos made his first-team debut during an away Union of European Football Associations Champions League fixture against Tottenham Hotspur during which he gave away a penalty for a foul on Luka Modrić in a 3–0 loss. On 9 November 2013, he scored his first Bundesliga goal against Schalke in an away match, which ended as a 3–1 defeat for Bremen.
He was loaned out to Union Berlin on 28 January 2016.