The blonde bombshell flaunted her assets in the racy dress, which featured long sleeves and a short floaty skirt.
Rachel flaunted her legs under black tights and heels as she did the letters and numbers for the show.
The wraparound design of the dress highlight Rachel's famous figure while she chatted with the guests and Susie Dent.
Viewers of the show rushed to compliment Rachel on her outfit, as one posted: "Rachel you're my girl crush absolute goddess."
Another added: "Loving watching Rachel Riley and Susie Dent."
A third chipped in: "Loving your dress."
Rachel also stole the show in a plunging pink dress yesterday which featured an embellished floral design of pink, red, white, and purple.
The skintight dress flaunted Rachel's famous derriere, which has previously won her Rear of the Year.
One viewer complimented her dress, posting on social media: "Love your dress on countdown today Rachel!"
Another added: "Rachel looks like an absolute dream."
Countdown continues tomorrow at 2.10pm on Channel 4.