Australia-born Johnson, who led Wales for three matches at the RBS 6 Nations Championship in 2006 and was an assistant to Mike Ruddock when Wales won the Grand Slam the year before, is now director of coaching at the Ospreys.
He will work alongside Cardiff Blues director of coaching David Young with the Barbarians, who will serve as World Cup warm-up opponents for Warren Gatland's Wales side.
Johnson said: "It's been a while since I've coached at the Millennium Stadium but the place is full of fantastic memories for me.
"I had a great time as Wales coach and the 2005 Grand Slam has got to be the highlight of my international coaching career.
"We played just two games at home, but that final decider against Ireland was just one of those epic games and something that will stay with everyone who experienced it forever.
"The Barbarians are steeped in tradition, they come with their own style and attitude and with so many experienced international players the side would probably coach itself, but it's an exciting prospect and a privilege to be involved.
"I'm looking forward to meeting up with some great players who I have a huge amount of respect for."
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