The 25-year-old moved to The Valley on an initial one-month deal and the Addicks have not conceded a goal in any of the matches he has played in.
Seaborne made his debut in a 2-0 win at Bristol City and also helped Charlton to wins over Burnley and Peterborough.
He then played in the goalless draw at Millwall but missed last Saturday's 2-2 draw against Brighton through illness.
Striker Leon Clarke is back with the Addicks after completing his loan spell at Scunthorpe, and goalkeeper John Sullivan has also returned to south-east London following his stint with Colchester.
Midfielder Eggert Jonsson, meanwhile, has gone back to Wolves at the end of his month-long spell with Charlton.