The 20-year-old, who has made seven appearances for Liverpool this season, has been training with the Addicks and is expected to sign for the League One club within 24 hours.
Charlton chairman Michael Slater told the club's official website: "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 has already played Premier League and European football this season, so he'll be a fantastic addition to our squad."
Ecclestone will be available to make his debut against Sheffield Wednesday on Saturday.
Source: PA
Source: PA