NASCAR Nextel Cup champion Johnson, 31, was ruled out of the event at the Stade de France after falling off a golf cart in California and breaking his wrist last weekend.
And Speed, 23, who races for the Toro Rosso team, was delighted, if a little surprised, to receive the call-up.
"It was a surprise to be invited to say the least," said Speed.
Race of Champions 2006
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"I definitely wasn't expecting to replace Jimmie Johnson in The Race of the Champions this year. Of course I'm really excited and I can't wait to race against all these guys."
Speed will take part in the main Race of Champions by himself as well as teaming up with US Rally champion Travis Pastrana at Team USA in the 'Nations Cup' tournament earlier in the afternoon.
He added: "I'm also really happy to be getting back together and teaming up with my old friend Travis, who I know from the Red Bull days."
Speed's announcement means three Formula One drivers will take part. David Coulthard and Heikki Kovalainen are the others.
You can see the entire event in LIVE video streaming on eurosport.com from 16:00 CET.
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