The 25-year-old forward has been signed for the second half of the Womens Super League season, which will resume for Blues on 29 June at home to Arsenal.
"It further outlines our intentions and vision of what we want to achieve at the club," said manager David Parker.
Blues fought off competition from some major European sides to sign Linden.
"It's a signal of the development the club is making not just in England but also in Europe that we are now attracting the top European players," added Parker.
She is the third German to join this season following the signing of Corina Schroder from Liverpool and the arrival in April of defender Marisa Ewers from Bayer Leverkusen.
On the back of two wins in three matches, Blues have climbed to third in WSL 1, seven points behind leaders Manchester City.
Linden said: "The conversations with David, (assistant manager) Marcus Bignot and (chief executive) Tim Andrews convinced me about the project and ambitions at the club.
"I'm looking forward to the new challenge to play in another country. I have always wanted to play in England."
Linden, who played at SGS Essen and Bayer Leverkusen before joining Frankfurt for the 2015-16 season, has represented Germany at every level.
Birmingham City Ladies have signed Germany international Isabelle Linden from Champions League holders FFC Frankfurt.