Burstow will, however, return to the Valley on loan for the remainder of the 2021/22 season.
Chelsea stated this in a statement on its website on Tuesday.
The statement read: “We have completed the signing of Mason Burstow from Charlton Athletic, with the teenage striker returning to the Valley on loan for the remainder of the 2021/22 season.
“Burstow will continue his development back on loan with Johnnie Jackson’s side, who are currently 14th in League One.”
Burstow, 18, joined Charlton in October 2020 as a second-year scholar from Welling United.
The England youngster made his senior debut earlier this season in the EFL Trophy against Crawley Town.