The 30-year-old actress - who won Best Supporting Actress at the ceremony on Sunday - surprised onlookers when she arrived in a pale pink Prada gown just hours after Valentino released a statement saying they had dressed the 'Les Miserables' star for the high profile event.
Anne explained that she "deeply regrets any disappointment caused" but decided to wear something different after she heard another actress, rumoured to be her co-star Amanda Seyfried, was wearing something very similar to her original choice.
In a statement to People.com, she said: "It came to my attention late Saturday night that there would be a dress worn to the Oscars that is remarkably similar to the Valentino I had intended to wear, and so I decided it was best for all involved to change my plans.
"Though I love the dress I did wear, it was a difficult last minute decision as I had so looked forward to wearing Valentino in honour of the deep and meaningful relationship I have enjoyed with the house and with Valentino himself. I deeply regret any disappointment caused."
Italian designer Valentino, 80, is a long-time friend of the actress and designed the custom pink and white wedding dress she wore for her Italian nuptials to Adam Shulman in September.
Anne was styled by celebrity fashion guru Rachel Zoe for the event, who also reportedly helped Best Actress winner Jennifer Lawrence select her Christian Dior Haute Couture gown.