The defender signed for Chelsea last summer from Strasbourg, featuring for the Blues in the Club World Cup, before being loaned back for the first-half of the season.
"I’m very happy to be here," Sarr told Chelsea's website.
"It was easy to come back because I know the guys, I know all the people, so it was easy. They are great players and it is exciting for me to be here.
On being reunited with his former Strasbourg coach Liam Rosenior at Chelsea, Sarr also stated: "I know Liam, he knows me,’ Sarr continued. ‘I know how he trains, I know how he prepares for the game. I know him very well. I think he’s a great coach.
"I think I progressed a lot at Strasbourg because I was the vice-captain, so it was another responsibility there.
"I felt great to be captain there, so that made me grow up from the experience. I prefer to lead by example. I don’t speak too much, I’m a very calm guy."