A US fashion brand’s stock has soared after the company announced an advertising campaign with actress Sydney Sweeney.

American Eagle’s stock jumped 12 per cent after the company announced the Euphoria actress would be featured in their new campaign.

The surge in stock added nearly $US228 million ($346 million) to its market capitalisation.

Sweeney announced her partnership with the brand in an Instagram post yesterday, which now has over 1 million likes. 

Company shares closed more than 4 per cent higher. Financial analysts called the share price jump “untethered from reality.”

The brand’s shares were down almost 50 per cent this year, prior to this week’s pandemonium over the shares.

Sweeney will have her own jeans called “The Sydney Jean” as part of the brand’s winter denim line, which will also include a limited-edition jacket.

The actress rose to fame after roles in HBO’s Euphoria and Netflix’s The White Lotus.

Sweeny has become one of the most recognisable actresses in the US.

She recently broke the internet after releasing a line of soap, which contained a small amount of her bathwater.

The actress has been a part of many advertising campaigns, including for brands like Ford, Baskin-Robbins and luxury fashion brand Miu Miu.

Sweeny has over 25.5 million followers on Instagram.