The blonde bombshell, 33, donned a bright red dress which featured an embellished silver shoulder piece over her chest.
Rachel looked stunning in the figure-hugging dress as she did the numbers and letters for the show.
It didn’t take long for viewers to comment on Rachel’s gorgeous choice of outfit, as one posted: “Doesn’t Rachel look amazing?”
Another added: “Rachel is so sparkly.”
A third wrote: “A little vintage look from Rachel tonight.”
This isn’t the first stunning outfit Rachel has worn in recent weeks, as she showed off her blossoming baby bump in a backstage picture.
Meanwhile, following her introduction, Jimmy joked about Rachel’s rd dress and then asked her about her support of Manchester United.
Rachel then recalled a story about a time she was met with some supporters, who took the opportunity to tease her about taking over Carol’s role.
The Countdown star revealed she was on the tube at the time, with the chant going: “Rachel Riley is a red is a red.
“Rachel Riley is a red, she hates Carol,” the mathematician said, giggling as Jimmy burst into laughter.
Countdown continues on Monday at 2.10pm on Channel 4.