The guinea is a coin of approximately one quarter ounce of gold that was minted in the Kingdom of England and later in the Kingdom of Great Britain and the United Kingdom between 1663 and 1814.

It was the first English machine-struck gold coin, originally worth one pound sterling, equal to twenty shillings; but rises in the price of gold relative to silver caused the value of the guinea to increase, at times to as high as thirty shillings; from 1717 until 1816, its value was officially fixed at twenty-one shillings. Following that, Great Britain adopted the gold standard and guinea became a colloquial or specialised term.

The name came from the Guinea region in West Africa, where much of the gold used to make the coins originated. Although no longer circulated, the term guinea survives in some circles, notably horse racing, and in the sale of rams, to mean an amount of one pound and one shilling (21 shillings) or one pound and five pence in decimalised currency.

Laureate head of King George III facing right; around, GEORGIVS III DEI GRATIA.

At centre, a shield set within the Garter, crowned; the shield is quartered with the arms of England, Scotland, France and Ireland with inescutcheon the arms of the Electorate of Brunswick and Luneburg surmounted by the Electoral cap; motto on the Garter; HONI SOI QUI MAL Y PENSE; the base of the Garter divides the date 1813; around, BRITANNIANUM REX FIDEI DEFENSOR.