The Norwegian's deal had long been anticipated, but was only formally announced at the VW WRC launch in Monaco on Saturday.
Mikkelsen contested a partial WRC season with VW's preparatory Skoda Super 2000 programme this year, alongside winning his second straight Intercontinental Rally Challenge title.
"It's been a few years in the IRC, so it's great to be back in the WRC with a World Rally Car," said Mikkelsen.
"I think it was 2008 the last time I drove one, so it's fantastic to be back.
"I am so happy to be with a great team and with a great car, I'm really, really positive about the future."
Sebastien Ogier and Jari-Matti Latvala will form VW's full-time 2013 WRC line-up.
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