Eccleston, who made his first start for the Reds in the Europa League against Utrecht, trained with the League One side on Wednesday morning.
Eccleston will stay at Charlton until May 31 and there is no recall allowed.
The 20-year-old hopes to make his Addicks debut against Sheffield Wednesday on Saturday.
Eccleston had a spell on loan at Huddersfield Town last season and Charlton are delighted to get their No.1 target.
Charlton Chairman Michael Slater said: "A number of clubs were keen to sign Nathan on loan during the transfer window, most of which were in higher divisions, so we had to move very quickly and we're delighted to have captured his signature.
"He's very highly rated and he'll be a fantastic addition to our squad."