The former VfB Stuttgart man leaves the Lilies on a free, with his contract expiring this summer. He has penned a three-year deal at the RheinEnergieStadion.
An experienced head
Rausch is well accustomed to the Bundesliga scene, with his 30 appearances for Darmstadt this season taking him past the 200 mark.
He also turned out for Stuttgart and Hannover 96, and was a regular in the UEFA Europa League with the latter - featuring 22 times across two seasons.
The now 26-year-old played 180 games for the HDI-Arena outfit, before sealing a switch to Swabian in 2013.
However, his time down south wasn't nearly successful and he was heading to Darmstadt just two years later.
His spell with the newly-promoted side saw him return to much more familiar form and help them secure their place in the Bundesliga next year.
Rausch will now be hoping to use that as a springboard at his new club and kick on to even greater things, with plenty of time left in his career.
Stöger, Schmadtke and Rausch have their say
“In Koka Rausch we are getting a quick and flexible player, who can use his strong left foot to set up teammates and strike dangerous set-pieces," said Peter Stöger.
The head coach was understandably pleased to have signed him, "We are happy that he has decided to join FC.”
General Manager Jörg Schmadtke also spoke to the club website, citing "many clubs" approached him but that the Billy Goats "clearly had some good arguments".
"Following a great year in Darmstadt I am incredibly excited about the next challenge in Cologne," admitted Rausch.
He added, "The club has made positive developments in recent years and I want to play my part to ensure that this development can continue."