Traffic police spotted the driver, 36, on the M4 motorway using dental floss to clean her teeth in her rear view mirror.
She was pulled over, lectured about careless driving and given a fixed penalty notice of £60.
Police say it is the latest example of people risking lives for a moment of high-speed vanity.
A spokesman said: "We've caught women applying lipstick, make-up and checking their hair in the mirror - and even men using an electric razor on their way to work.
"But this is the first time we've seen someone flossing their teeth at the wheel. It was a quite bizarre sight.
"It is particularly dangerous because you need to use both hands which clearly should be used to drive the car."
Experienced traffic police often say some people's driving is mental - but one joked: "On this occasion it was dental."
The woman was caught by Gwent Police near junction 24 of the M4 at Coldra, Newport, during an eight-day clampdown.
Officers in an unmarked police car caught 57 drivers committing offences on a stretch of the M4.
Only one was caught cleaning her teeth - and she accepted the fixed penalty fine instead of fighting the case.
Other offences included driving without a seatbelt, using a mobile phone, vehicle defects and a learner driver towing a load.
Inspector Lee Ford said: "It is very concerning that a number of drivers are driving irresponsibly and carelessly on our motorways.
"The potential risks not only risk the safety of the driver involved but also other road users."