The 29-year-old returns to Belgium after previously making more than 100 appearances for Mouscron and Charleroi.
Last season, Koffi spent time on loan at Ligue 1 side Angers, establishing himself as their first-choice goalkeeper and keeping 10 clean sheets.
"I am very happy and proud to become a Union player today. I have played against the club several times in the past and it always felt like a real family to me,” Koffi told club website.
“I'm delighted to now be part of that family. This is a club with a wonderful history, but also an impressive story over the past few years.
“That really motivates me and I hope I can make my own contribution here. Union is the right step in my career at this moment.
“I don't want to skip any steps and I know there is still a lot for me to learn. At the same time, I have gained valuable experience over the years, and I hope to share that with my teammates as well."
Koffi began his European career in Belgium and also played alongside current Union midfielder Adem Zorgane during his spell at Charleroi.