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."
Johnson scored 85 goals in 160 appearances during his first stint at the club, achieving promotion to the top flight in 2004.
He finished the following season as the top-scoring Englishman in the Premier League and was called into the England squad by then-manager Sven-Goran Eriksson.
Johnson went on to win eight international caps and is one short of 150 career goals in club football.
