The centre-back joins the club from FC Rosengård as Nick Cushing seeks to bolster his squad, with the Blues having now entered the second half of their debut season in the FA Women's Super League.
Also known as 'Paula', Radtke helped her side to the Damallsvenskan title last year. Bringing with her a wealth of domestic experience, the 29-year-old is excited to be part of MCWFC's adventure in the top flight.
"It feels really good to be signing for Manchester City," she declared. "There's a big story behind the club and a big future ahead that will bring good things.
"I'm curious to find out how the style of play here differs to what I have experienced before.
"As a player, I always give 100% and I'm strong, powerful and I like to be creative. Hopefully, I can bring some goals and some assists and I hope I can show my experience and share it with the team.
"In Sweden, I was one of three Germans to win the league and I want to be the first one to win a trophy with Manchester City."
Having joined Hallescher FC from CFC Germania Köthen, Radtke was named Sportswoman of the Year in 2001 and also secured a European Championship for her national side at under-17 level.
She then transferred to 1. FFC Turbine Potsdam where she became a German champion and German Cup winner in 2004, before re-joining CFC Germania Köthen's youth academy. The defender then returned to professional football with FC Lok Leipzig, FF USV Jena and Swedish side FC Rosengård.
Reflecting on his latest capture, manager Cushing asserted: "We are delighted to add Paula to our squad.
"She is a player that adds balance and further strengthens us as a team.
"We look forward to her joining the rest of the players on the training pitch, so we can keep improving our playing style."