The comedy panel show previously merged with the popular daytime quiz as part of Channel 4's Mash Up in January.
Jimmy Carr will again host the special episode, with team captains Sean Lock and Jon Richardson facing off in a rematch.
Irish stand-up comic David O'Doherty will join Countdown regular Susie Dent in Dictionary Corner, while Rachel Riley will receive "a little help" from comedian Joe Wilkinson on numbers and letters.
The new 8 Out of 10 Cats Does Countdown will air as part of Channel 4's Funny Fortnight, which will feature two weeks of new Channel 4 comedies throughout August.
Other Funny Fortnight commissions include new shows from Harry Hill and Vic and Bob, and Facejacker chat-show spinoff Verry Terry starring Kayvan Novak.
> Frankie Boyle, Vic & Bob, Harry Hill for Channel 4 comedy season
> Jimmy Carr's tax shame sends 8 Out of 10 Cats ratings flying
Watch a clip from 8 Out of 10 Cats below: