The Koh-i-Noor (Persian for 'Mountain of Light'; KOH-in-OOR), also spelled Kohinoor and Koh-i-Nur, is one of the largest cut diamonds in the world, weighing 105.6 carats (21.12 g). It is part of the Crown Jewels of the United Kingdom. The diamond is currently set in the Crown of Queen Elizabeth The Queen Mother.

There are multiple conflicting legends on the origin of the diamond. However, in the words of the colonial administrator Theo Metcalfe, there is "very meagre and imperfect" evidence of the early history of the Koh-i-Noor before the 1740s that can directly tie it to any ancient diamond. There is no record of its original weight, but the earliest attested weight is 186 old carats (191 metric carats or 38.2 g). The first verifiable record of the diamond comes from a history by Muhammad Kazim Marvi of the 1740s invasion of Northern India by Nader Shah. Marvi notes that the Koh-i-Noor as being one of many stones on the Mughal Peacock Throne that Nader Shah looted from Delhi. The diamond then changed hands between various empires in south and west Asia, until being given to Queen Victoria after the British East India Company's annexation of the Punjab in 1849, during the reign of the then 11-year-old Maharaja of the Sikh Empire Duleep Singh, who ruled under the shadow influence of the Company ally Gulab Singh, the first Maharaja of Jammu and Kashmir, who had previously possessed the stone.

Originally, the stone was of a similar cut to other Mughal-era diamonds, like the Daria-i-Noor, which are now in the Iranian Crown Jewels. In 1851, it went on display at the Great Exhibition in London, but the lackluster cut failed to impress viewers. Prince Albert, husband of Queen Victoria, ordered it to be re-cut as an oval brilliant by Coster Diamonds. By modern standards, the culet (point at the bottom of a gemstone) is unusually broad, giving the impression of a black hole when the stone is viewed head-on; it is nevertheless regarded by gemologists as "full of life".

Since arriving in the UK, it has only been worn by female members of the family. Victoria wore the stone in a brooch and a circlet. After she died in 1901, it was set in the Crown of Queen Alexandra. It was transferred to the Crown of Queen Mary in 1911, and finally to the Crown of Queen Elizabeth The Queen Mother in 1937 for her coronation.

Today, the diamond is on public display in the Jewel House at the Tower of London. The governments of India, Iran, Pakistan, and Afghanistan, as well as the Taliban, have all claimed ownership of the Koh-i-Noor, demanding its return ever since India gained independence from the British Empire in 1947. The British government insists the gem was obtained legally under the terms of the Last Treaty of Lahore and has rejected the claims.