Casteels was signed by VfL Wolfsburg from TSG 1899 Hoffenheim in January 2015, but spent the first six months of the three-and-a-half-year contract on loan at Werder Bremen. He played for Wolfsburg in the 2015 DFL-Supercup, saving from Xabi Alonso in the penalty shootout as his team won after a 1–1 draw.