Former Germany Under-23 international Ewers, 27, joined Leverkusen in 2012 after six years with Hamburg.
"I am delighted that we have been able to bring Marisa to the club,"  manager David Parker told the club website.
"She has been one of the most consistent performers in the league for the past eight seasons."
Details about the length of Ewers' contract have not been disclosed.
Birmingham are currently bottom of the WSL 1 table following defeats in their opening two games of the new season.
Women's Super League One side Birmingham City have signed defender Marisa Ewers from Frauen Bundesliga club Bayer Leverkusen.