Maths whizz Riley, 35, spoke about the unpleasant moment filming the daytime quiz when she had been introduced to the male fan.
She told The Sun: "This married couple came up to me and the lady said 'oh my husband really likes watching you, do you mind if we just…' and he squeezed my arse.
"I didn’t know what to do with myself. I was a bit shocked and stood back and they didn’t get invited back to the audience.
"You get the odd a**e grabber."
Riley is taking a break from Countdown as she is on maternity leave and currently expecting her second child with former Strictly Come Dancing partner Pasha Kovalev.
The couple, who married in Las Vegas in 2019, already have daughter Maven together, who will turn two in December.
In a recent appearance on Channel 4 TV views show Complaints Welcome, Riley also spoke about her Countdown wardrobe.
The star admitted that she had not been happy with the frumpy outfits she was given to wear when she first joined the show in 2009.
She said: "I had to buy some new clothes at one stage.
"I just didn’t know what to wear and they were trying to dress me more like my mother at that stage, everything had to be below the knee."
Riley will be temporarily replaced by Dr Anne-Marie Imafidon while she is on maternity leave, who will join the all-female Countdown line-up of host Anne Robinson and dictionary corner's Susie Dent.
When she announced her second pregnancy, Riley used a Countdown conundrum to share the happy news.
She posted a photo of herself with the quiz show's letters board spelling out "FITTRILEY", and anagram for "fertility".
Riley captioned the snap: "A special teatime teaser! And your clue is... Riley had this to thank for the reason her clothes no longer fit her... And it’ll only get worse till Autumn!
"Happy, happy days for our little family @pashakovalev and future big sister Mave."
Watch: Countdown letters spell out 'poo anally' on Rachel Riley's 3000th episode