"I am devastated to announce that I will not be able to compete in Sochi," Vonn said on her Facebook page. "I did everything I possibly could to somehow get strong enough to overcome having no ACL but the reality has sunk in that my knee is just too unstable to compete at this level."
The world champion downhill skier said on her post she is having surgery soon to be ready for the World Championships in her hometown of Vail next February.
"On a positive note, this means there will be an additional spot so that one of my teammates can go for gold," Vonn wrote. "Thank you all so much for all of the love and support. I will be cheering for all of the Olympians and especially team USA!"
Under Armour released a statement Tuesday, saying: "Lindsey Vonn is one of the most iconic athletes of this generation and we are honored that she is such an integral part of the Under Armour family and an exemplary ambassador for the brand. Her strength, drive, and relentless will to compete despite severe injury are reflective of her true love for the sport and her undeniable spirit. We wish Lindsey a speedy recovery to full health and look forward to seeing her back out on the hill for the continuation of her remarkable career."
Vonn, who has been featured in recent Under Armour advertising campaigns, was to be one of the individual Olympians sponsored in the winter games by the Baltimore-based sports apparel maker.
The skier was shown battling extreme conditions while wearing Under Armour cold weather apparel in the "Under Armour Makes You Better," commercial, launched in November to highlight new apparel and the company's Olympic sponsorships, such as the U.S. bobsled and speedskating teams.