The South Korea international has signed a two-year deal with the Bundesliga newcomers, arriving on a free transfer.
Cha previously played for Arminia Bielefeld, Eintracht Frankfurt, Mainz and Freiburg in Germany's top two divisions, totalling 115 top-flight appearances before moving to Scotland.
"I am delighted to be back in the Bundesliga," said the 31-year-old on his new club's website.
"Fortuna are a great club with great fans and I am looking forward to the challenge and the atmosphere in the Dusseldorf Arena."
Fortuna's director of sport Wolf Werner hopes Cha's Bundesliga experience will serve them well in their first season back in the top flight in 15 years.
"He has already proven his potential in the Bundesliga in the past," he said. "Du-Ri Cha gives us a versatile and internationally-experienced player for the right-hand side of the field."