He has now joined Alan Pardew's side's promotion push and is delighted to be at The Valley.
"I'm really excited to be here," said Sinclair.
"I've come here to get more experience, and I want to keep playing and get some goals for the club.
"There were a few clubs in for me, but I thought Charlton would be the right one for me and I'm sure I've made the right choice."
"I want to get some more games on board and then I can hopefully return to Chelsea in the summer after helping Charlton go up." he added.
Sinclair's loan deal means that he will be able to play in the Championship play-offs, if that is the route the Addicks season takes.
He also goes straight into the squad for Saturday's clash with Sheffield United at Bramall Lane.