The watchdog upheld a complaint that the ad was offensive and irresponsible as the girl appeared to be under 16 years of age.
American Apparel said the ad was intended to appeal to women, and that the model was shown to be in a relaxed "home" environment, wearing the hoody as a consumer might.
The retailer told the ASA the model was 23. It did not believe the ad to be gratuitous as the model remained clothed, wearing the hoody and a pair of shorts. The ad was intended to focus on the hoody as opposed to the model, and she was not portrayed as a sex object or in a negative light, it said.
The ad appeared in Vice magazine, which the retailer understood was targeted at 18- to 34-year-olds. It had previously advertised in Vice with similarly styled images.
The ASA considered that although the model was 23, the natural and minimal styling of the ads made her appear under 16.
The watchdog concluded that the ad was inappropriate and must not appear again in its current form.