The Blades have the right too recall him after the first 28 days of the loan spell, and the two clubs will discuss nearer the time whether he will be able to play for the Addicks in the third round of the FA Cup on the day his loan is due to expire.
The news will please Northern Ireland manager Nigel Worthington who has expressed concern that several of his squad are not playing regular first-team football.
Gillespie has not started a game this season for Sheffield United, who are fourth in the Championship - 18 places above Charlton.
The Addicks are searching for a new manager following Alan Pardew's departure at the weekend after they lost 5-2 to Sheffield United.
Phil Parkinson has taken temporary charge for Tuesday evening's game with Queens Park Rangers.
Gillespie told BBC Sport that he had turned other offers from "lower down" before plumping for the loan move to Charlton.
"I didn't want to go out of the Championship," added the Northern Ireland player.
"It was nice that this chance came along for me, late last night.
"It will help my fitness and I know that Nigel (Worthington) wants us all playing regular first team football."
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