The 27-year-old, who was born in Hamburg, began her career at Hamburger SV in 2006.
She later moved onto Bayer Leverkusen in 2012, where she was their club captain over the past two seasons.
Ewers moves to England, following eight years in the Frauen Bundesliga.
Birmingham City manager David Parker said: “I am delighted that we have been able to bring Marisa to the football club. This signing shows the direction and vision of the Club as we move forwards on every level.
“Attracting a player of Marisa’s ability and stature from the Frauen Bundesliga shows the calibre of players we are starting to attract with our vision and project for the future.
“She has played at every level for her country up to the Under-23s and has been one of the most consistent performers in the league for the past eight seasons since making her debut at 18.
“It was not just her ability that was a key factor in bringing her to the Club, but we are also bringing in a leader and a player who has been a professional in one of the world’s best leagues which will help develop our Club’s culture and have a major impact on our young players. I cannot wait to start work with her come June.”
Source – www.birminghamcityladiesfc.co.uk