Former Barcelona goalkeeper Victor Valdes is training with Manchester United as he recovers from a serious knee injury.
Crystal Palace defenders Damien Delaney and Joel Ward have yet to agree new deals at the club.
Johnson, 30, who had been expected to join back up with former boss Dave Jones last week, will stay at Hillsborough for three months.
Jones, who worked with the centre-half during his time in charge at Cardiff, told the club's official website: "We are pleased that Roger is here because he is a fantastic talent.
"I know the lad well, he did fantastic for me at Cardiff, he is a very good player and we have been given the opportunity to bring him here and we are grateful for that.
"He knows what my principles are and what I expect from my players and he'll know he'll have to fight for his place.
"We have got a very, very good player, he has the stage here to perform and it's up to him to go out and there take this opportunity."
Johnson, who started his career at Wycombe, was signed by then Cardiff manager Jones in 2006 and went on to win the club's player of the year award twice during his three-year stay before Birmingham paid £5million for him in 2009.
Wolves paid £7million to take Johnson to Molineux on the eve of their stint in the Barclays Premier League in July 2011.
But Johnson has not had the best of times at Wolves and this summer was one of four players transfer-listed by manager Kenny Jackett and forced to train with the under-21s squad.