Goal.com exclusively revealed on Friday that Cottagers boss Martin Jol was close to brining the 35-year-old to the club as he looks to boost his options following the departures of Moussa Dembele and Clint Dempsey.
Karagounis is Greece's captain and has made 120 appearances for his country, while enjoying club spells at Panathinaikos, Inter and Benfica.
|17/2||Fulham are 17/2 with Bet365 to beat West Brom 2-0
"I'm looking forward to working under Martin Jol and playing alongside some fantastic players at this club."
The Greek's new manager, Jol, added: "Giorgos Karagounis has a wealth of club and international experience and I'm happy that he's joined us for this season.
"He's been an influential player for his club sides and is a renowned player in world football as he's about to become the most-capped player for Greece. He brings many certain qualities to our squad."
Fulham have enjoyed a mixed opening to the new season, winning their first game 5-0 at home to Norwich City before recent losses to Manchester United and West Ham.