The 35-year-old left back will drop down a division from the Premier League after joining the Hammers until the end of the season. He spent the last three months on loan at Upton Park.
"Chris did fantastically well for us in his six years here and I don't think we can forget what a great servant he's been. He won five England caps with us," Charlton manager Alan Curbishley told reporters.
"But he needs first team football and he's been itching to get that for months. When West Ham came in he wanted to go, and we thank him for everything he's done for us."
