Trans-fats are used in many fried and baked foods
California has become the first US state to ban restaurants and food retailers from using trans-fats, which are linked to coronary heart disease.
Governor Arnold Schwarzenegger said the new legislation, which will take effect in 2010, represented a "strong step toward creating a healthier future".
Violations will incur fines of between $25 (£13) and $1,000 (£502).
Trans-fats are chemically altered vegetable oils, used to give processed foods a longer shelf-life.
Some cities, like New York City, Philadelphia and Seattle, have already banned the fats. Many food makers and restaurant chains have also been experimenting with replacements for oils and foods that contain them.
Trans-fats are produced artificially in a process called hydrogenation which turns liquid oil into solid fat.
They can be used for frying or baking, or put into processed foods and ready-made mixes for cakes and drinks like hot chocolate.
Trans-fats are used because they are cheap, add bulk to products, have a neutral flavour and give products a long shelf-life. They have no nutritional value.
They are partially hydrogenated vegetable oils, turning oily foods into semi-solid foods
Used to extend shelf life of products
Put into pastries, cakes, margarine and some fast foods
Can raise levels of "bad" cholesterol
Even a small reduction in consumption can cut heart disease
They have no nutritional benefit
The US Food and Drug Administration estimates that on average, Americans eat 4.7lb (2.14kg) of trans-fats each year.
A review by the New England Journal of Medicine in 2006 concluded that there was a strong connection between the consumption of trans-fats and coronary heart disease. It found they boosted "bad" cholesterol levels in the body.
The review said that eliminating artificial trans-fats from the food supply could prevent between six and 19% of heart attacks and related deaths each year.
The legislation signed by Mr Schwarzenegger will ban from 1 January 2010 the use of trans-fats in oil, shortening and margarine used in spreads or for frying.
The president of the California Academy of Family Physicians, Jeffrey Luther, said that the law,
"when it finally takes effect, will be a tremendous benefit", adding that there was no safe level of consumption, as with cigarettes.The California Restaurant Association opposed the ban, but a spokesman said that it had no plans to challenge it in the courts, in part because some restaurants have already begun to phase out trans-fats to satisfy customers.