The songstress - who released a three-track EP as a Christmas present for fans this week - launched her search for an ethical design last month. Lewis judged the entries alongside Glamour magazine editor Jo Elvin and designer William Tempest.
The winning dress, created by Lilyana Maslenishka from Bulgaria, was showcased by Lewis during a performance on the Royal Variety Performance - which was filmed in Manchester earlier this month.
"We all spent so long going through each and every design and I was amazed and impressed by the care, time and effort people put in," the singer said. "Thank you to Jo and William for their invaluable help in judging all of the entries. Lilyana is such a worthy winner and I absolutely loved wearing her dress."
The judges also named designs by Josie Smith and Francisco Aguilar as first and second place runners-up respectively.
Lewis performs her cover of Nine Inch Nails' 'Hurt', currently number three on UK iTunes, on the Royal Variety Performance tomorrow night from 7.30pm on ITV1. The Hurt: EP is available to download now.
> Leona Lewis interview: 'I want to give people the best of me'
Watch Leona perform 'Hurt' on X Factor: