The experienced South Korea international returns to the country of his birth on a free transfer after spending the past two years in Scotland
The 31-year-old full-back returns to the country of his birth on a free transfer, having spent the past two years in Scotland with champions Celtic.
Cha, who previously played with a number of German clubs including Leverkusen, Eintracht Frankfurt and Freiburg, said he was delighted to return to the Bundesliga.
“I am happy to be back in the Bundesliga. Fortuna is a great club with great fans,” Cha told the club's website.
Cha struggled to cement a first-team place with Celtic, making 44 appearances in his two seasons at the club.
"Everyone at the club would like to wish Cha every success in his future career," Celtic said in a club statement.