The former Tottenham and Leicester left-back, 31, has agreed an 18 month deal at SE7 and is manager Alan Pardew's second signing of the January transfer window, after the arrival of Chinese captain Zheng Zhi.
Pardew said about his new signing, "We need to be creative in the transfer market and I think what Ben brings is a strength of character and a determination to get us out of trouble, and that's what we need now".
Pardew also said that he had no reservations over Thatcher's poor disciplinary record and he would go straight into the side for Saturday's massive home game against Middlesbrough.