The Strictly Come Dancing runner-up and opera singer follow stars such as Denise Van Outen, Sian Lloyd and Robbie Williams, who have won the award in previous years. Last year's winners were glamour model Jennifer Ellison and soap star Ryan Thomas.
"I'm very flattered and I'm now going to insure each cheek for a considerable sum of money," said ex-S Club 7 star Stevens.
Watson joked: "For the past ten years it's always been about The Voice but, for the first time, I'm delighted to say it's about my rear. Winning the Rear of the Year Award is better than getting an OBE."
Event organiser Tony Edwards told PA: "This is the era of the well-toned rear when people are taking care of their bodies with work-outs, jogging and aerobics. Rachel and Russell typify the look."
The Rear of the Year competition is now in its 27th year.