The actress, who dated Lord of the Rings star and Lost castmate Dominic Monaghan, will play elf character Tauriel in director Peter Jackson's fantasy epic The Hobbit.
In a post on his Facebook.com page, Jackson writes, "Her name means 'daughter of Mirkwood' and, beyond that, we must leave you guessing!
"What is not a secret is how talented and compelling an actress Evangeline is; we are thrilled and excited she will be the one to bring our first true Sylvan Elf to life."
Jackson has also added Aussie comedian Barry Humphries and Brits Luke Evans and Benedict Cumberpatch to his cast in the past few days.
They join Martin Freeman, Hugo Weaving and Lord of the Rings regulars Sir Ian McKellen, Elijah Wood, Cate Blanchett, Orlando Bloom, Christopher Lee, Andy Serkis and Ian Holm in the film, which will be released in December, 2012.