Stevens is most famous for his role as Matthew Crawley in ITV1's smash hit period drama.
Jo Brand, Alexander Armstrong, Lee Mack, Stephen Mangan and David Mitchell have all guest hosted episodes in the most recent run of HIGNFY. Miranda Hart has already been confirmed for a stint in the host's chair later in the series.
Stevens said: "I'm hugely excited to be hosting this week! I've watched and loved the show for years - far longer than I've watched the actual news..."
Comics Miles Jupp and Susan Calman will join team captains Ian Hislop and Paul Merton on the panel show.
Have I Got News For You, which is now in its 42nd series, airs on Fridays at 9pm on BBC One.
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