The player, who has been on loan with the club all season, will join for approximately £1.2 million after Fulham agreed to pay the fee that was originally set when the player first joined.
Speaking on Twitter, Riether is thought to have thanked FC Koln for his time at the club, and wished them success in their bid to gain promotion back to the Bundesliga 1. German website, Kicker, confirmed the news.
It will come as a huge relief to Fulham fans, who have been stressing the importance of tying the 30 year-old down permanently. He has been invaluable in his first Premier League campaign, claiming the full back spot as his own despite pressure from the likes of Chris Baird, Stanislav Manolev and, before he left, Stephen Kelly.
His reliability has been his greatest asset - the German rarely makes mistakes and puts in consistently solid performances. This is rare for any player, and even more exclusive for a defender.
Yet, while his sturdiness has been vital, so has his exploitative nature. Riether has become an overlap master, forming a partnership with right winger Ashkan Dejagah that has proved exceptionally fruitful. He was recently awarded the Players Player of the Year award, too.
The only factor that does not work in his favour is, inevitably, his age, but few will have any qualms over that given his performances. There are many areas in the squad that require a hefty injection of youth but, with Riether where he is, right back is not one of those places.