The 20-year-old was expected to arrive at Stade August Bonal last week to undergo his medical, but touched down on Monday and passed all the tests ran on him before an official announcement was made.
The youngster has reunited with Sochaux boss Peter Zeidler, who previously coached him at Liefering until 2015. He is expected to fly out with the first-team squad for the summer tour within Europe as they hope to win qualification back to Ligue 1 next season.
"Great opportunity for me to make a name for myself and I believe everything will work to perfection here," Ati said.
"The manager wanted me here so when I was told about his interest, I never hesitated since I have previously worked with him in Austria. I'm aware of his style and hopefully things will work. It's a great feeling and I hope I will be able to help the club achieve the target they have set for the upcoming season," he added.
Ati, who spent last season on loan at FC Liefering, making 11 appearances, was part of Ghana's squad that participated in the 2015 Fifa Under-20 World Cup in New Zealand.