The 33-year-old, who previously played at Selhurst Park from 2002 to 2006, returns to the club on a short-term contract that will also see him take on some coaching responsibilities.
Johnson was released by QPR at the end of last season, but will now bolster manager Neil Warnock's attacking options after being included in Palace's 25-man Premier League squad.
"I am delighted to be coming back to a club that I love and have a real soft spot for," he said.
"I would like to thank the gaffer and the chairman for giving me this opportunity and I am looking forward to helping this football club have continued success in the Premier League.
"I have also been given an opportunity to help the youngsters in a coaching capacity, something I am really looking forward to getting involved in."
