With the necessary paperwork completed on Monday evening, the Blues announced the deal on Tuesday with the 18-year-old forward seeing out the season back at the Valley, on loan.
Burstow was left out of Charlton’s squad for Monday night’s 2-1 win over Portsmouth in order to conclude talks with Chelsea.
Addicks boss Johnnie Jackson said: “A bid has come in from a Premier League club for Mason, so we’ve allowed him to go and chat with them. His focus was going to be elsewhere tonight, so we felt it best to leave him out of the game.”
Chelsea were one of a host of top-flight clubs monitoring the teenager, but stole a march on their rivals with a “substantial” offer.
Jackson added: “It’s great that the chairman has got a deal he is happy with, but also we get the player back.”
Burstow has only made seven starts for Charlton this season but has scored six goals in 14 appearances.
