A hallmark is an official mark or series of marks struck on items made of metal, mostly to certify the content of noble metals—such as platinum, gold, silver and in some nations, palladium. In a more general sense, the term hallmark can also be used to refer to any distinguishing characteristic. It is a stamp to certify quality.

Hallmarks are often confused with "trademarks" or "maker's marks". A hallmark is not the mark of a manufacturer to distinguish his products from other manufacturers' products: that is the function of trademarks or makers' marks. To be a true hallmark, it must be the guarantee of an independent body or authority that the contents are as marked. Thus, a stamp of '925' by itself is not, strictly speaking, a hallmark, but is rather an unattested fineness mark.

Hallmarking started in the 1300’s and is the oldest form of quality control.
There are currently four Assay Offices in the UK (London, Birmingham, Sheffield, Edinburgh) and overseen by the British hallmarking Council. The oldest Assay Offices are working from Royal Charters (London and Edinburgh).