The defender departed Turkish Super League side Genclerbirligi this January after the expiration of his contract and had an unsuccessful trial with Spanish club Elche.
However, on Tuesday, the brother to Manchester United midfielder Paul, was handed a contract by the Five Stripes after delivering a convincing showing during pre-season.
In his reaction, club president and technical director Carlos Bocanegra is delighted with the addition of the 28-year-old and hopes he will help Frank de Boer’s men with his wealth of experience.
"We're pleased to officially welcome Florentin to our club," Bocanegra told club website.
“He's a physical centre back who also has the ability to play with his feet. He also brings an immense amount of experience after representing his club and country in various international tournaments and we expect him to compete for playing time on our backline."
The defender has 21 caps for Guinea since making his international debut against Mali in 2010.
He will look to make his first appearance for Atlanta against Los Angeles FC in Sunday’s friendly encounter.