Ostberg was part of Citroen's factory line-up in the 2014 and '15 seasons but has spent the last two years in M-Sport Fords - running his own privateer programme with one in '17.
He joins full-season drivers Kris Meeke and Craig Breen, who is sharing the second seat with Sebastien Loeb, at Citroen for Sweden.
"It's a small surprise to me as well - I'm really happy and really honoured to be part of the team for Raly Sweden," said Ostberg as he was unveiled as a Citroen recruit on stage at Autosport International during the 2018 WRC launch.
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"It's one of my favourite events and one I've been on the podium at many times so I'll try to do that again."
Ostberg confirmed that Sweden was currently a one-off appearance, but he hopes to earn further chances with Citroen later in the year.
"I feel really motivated and I hope that together we can deliver good results and have a few more appearances durng the year," he said.
"This team is a great team, I've worked with them in the past and I know what they stand for."
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