The 29-year-old has made two appearances for the German national team and joins the Cottagers from Cologne, who were recently relegated from the Bundesliga.
Riether has signed on loan until the end of the season and Fulham have an option to make the move permanent at the end of that period.
"I feel very pleased to have signed," Riether said. "It feels like I've had a long break from football and for me this is a new experience in a new country.
"Fulham is a good club, a big club with a big tradition and for me it's great to be here.
"I played here one time with Wolfsburg and I remember Craven Cottage being a great stadium with great fans, and I hope I can go on to have a great season here."