Starring alongside Nick Hewer from The Apprentice (best viewing duo ever), Myleene looked stupidly excited to be on the show as she posed for a snap on Twitter.
Perching herself in front of the big Countdown clock with her geek glasses firmly in place, the 35-year-old is probs the hottest thing to ever grace the infamous desk.
Myleene topped her geek-chic look off in a neon yellow Littlewoods dress from her own collection as she blabbed to her followers about her day.
Giving an insight into her excitement, she tweeted: "You will NEVVVVER believe where I am today. *high fives student self *
"Du duh, du duh, dududu duuuuh boooo."
She then tweeted a picture of herself with Nick during a break from filming, adding: "I love him soooo much. #countdown"
Myleene was previously tipped to replace Carol Vorderman on the show in 2008, becoming the favourite before Rachel Riley was brought in.
This afternoon, Rachel tweeted her excitement at working with Myleene, adding: "Best day of work EVER! Watch our finals with @KlassMyleene in June, I love our @C4Countdown team."
Meanwhile, Myleene has been keeping her bravest face on for the past week, following reports of a feud between her and her ex-husband, Graham Quinn.
Graham has reportedly requested the entire sale sum of the couple's former family home following their divorce.
But Myleene appears to be unfazed by all the drama, instead enjoying a sunshine holiday with her two adorable daughters before today's Countdown bonanza.
[ Related article: Myleene Klass shows off her gorgeous bikini body on holiday with her girls ]
[ Related article: Myleene Klass's ex-husband, Graham Quinn, asks for full sale price of their former home ]