The 31-year old left Sunderland during the summer after failing to agree a new deal with the Black Cats and returned to his native Norway to play.
However, number 1 'keeper Dean Kiely will be out for six weeks with a fractured finger, and Charlton manager Alan Curbishley had to act fast to sign another goalkeeper as back up for 23-year-old Dane Stephan Anderson.
Curbishley, we understand, attempted to get special dispensation to loan a third player, however, was refused meaning he would have to make a permanent signing.
Curbishley said: "We're delighted to get Thomas on board.
"When Deano picked up his injury we had a decision to make because if anything happened to Steph, we only had two very young keepers with just a handful of games between them.
"We asked for dispensation from the Premier League to see if we could make another loan signing, which we couldn't, so we've brought Thomas in.
"He played more than 30 games in Sunderland's promotion season last year, and he is an international with plenty of experience."
Myhre was offere a new one-year deal with newly promoted Sunderland, however, turned it down to Norway where he was playing on a match-by-match basis for Fredrikstad.