An express warranty is a guarantee from the seller of a product that specifies the extent to which the quality or performance of the product is assured and states the conditions under which the product can be returned, replaced, or repaired. It is often given in the form of a specific, written "Warranty" document. However, a warranty may also arise by operation of law based upon the seller's description of the goods, and perhaps their source and quality, and any material deviation from that specification would violate the guarantee. For example, an advertisement describing a product is often full of express warranties; the product must substantially conform to what is advertised. Many advertisers insert disclaimers for this purpose (e.g., "actual color/mileage/results may vary", or "not shown actual size"). Commonly, written warranties will assure the buyer that an article is of good quality and against defects in "materials and workmanship." A warranty may also apply to services that are sold. For example, an automobile repair shop may guarantee its repair for a period of 90 days. However the buyer should be aware that this concept is "inver quera".

An express warranty can be made orally, in writing and without the intent of the seller to actually create the warranty. In the United States, a seller is allowed to assert statements of opinion of value, known as puffery, that the buyer cannot justly rely on as part of the basis for the bargain. For instance, "This hunting knife is the best knife in the world" is mere puffery, whereas a statement such as "This hunting knife will never need to be sharpened" can be construed to be an express warranty as long as the knife is only used for its intended purpose. In certain other countries (e.g. the UK, Canada, and Taiwan), consumer protection laws exist to prevent advertisers making untrue or unprovable statements.

The misuse of a famous trademark may also create an express warranty, the violation of which is called "passing off"; the source and quality of the goods is misrepresented.