The Kayñ (; , ) were an Oghuz Turkic ethnic group and a sub-branch of the Bozok tribal federation. In his DëwÃÂn LughÃÂt al-Turk, the 11th century Kara-Khanid scholar Mahmud al-Kashgari cited as of one of 24 Oghuz tribes, saying that Oghuz were also called Turkomans.
The name Kayñ means "the one who has might and power by relationship" and a Turkmen proverb says that "the people shall be governed by Kayñ and Bayat tribes" ().
In his history work Shajara-i TarÃÂkima, the Khan of Khiva and historian, Abu al-Ghazi Bahadur, mentions among the 24 ancient Turkmen (Oghuz Turkic) tribes, direct descendants of Oghuz Khagan. Oghuz Khagan is a semi-legendary figure thought to be the ancient progenitor of Oghuz Turks. translates as "strong". In his extensive history work âÂÂJami' al-tawarikhâ (Collection of Chronicles), the statesman and historian of the Ilkhanate Rashid-al-Din Hamadani also says that the tribe comes from the oldest of Oghuz Khan's 24 grandchildren who were the patriarchs of the ancient Oghuz tribes, and the name Kayñ means "powerful".
Soviet Sinologist and Turkologist Yury Zuev based on the analysis of tribal names and tamgas from Tang Huiyao, identifies a number of ancient Central Asian Turkic tribes as Oghuz-Turkmen tribes, one of them is the Kay tribe, whom the Chinese knew as Xà奠(< MC *óiei). After examining Chinese sources & consulting the works of other scholars (Pelliot, Minorsky), Zuev proposes that the Kay had belonged to the proto-Mongolic Xianbei tribal union Yuwen Xiongnu and that Kay had been ethnic and linguistic relatives of the Mongolic-speaking Khitans, prior to being known as an Oghuz-Turkmen tribe by the 9th century. Likewise, Hungarian scholar Gyula Németh (1969) links Kayñ(ÃÂ) to the (para-)Mongolic Qay/XÃÂ, whom Tibetans knew as Dad-pyi and Göktürks knew as Tatabï; however, Németh's thesis is rejected by Mehmet Fuat Köprülü among others. Later on, Németh (1991) proposes that Mg. Qay is derived from Tk. root qað- "snowstorm, blizzard"; nevertheless, Golden points out that Qay has several Mongolic etymologies: ãai "misfortune", ÃÂai "interjection of grief", ÃÂai "to seek", ÃÂai "to hew".
Even so, Köprülü rejects scholarly attempts to link the formerly Mongolic Qay/Xi to the Oghuz Turkic tribe Qayñ(ÃÂ); he points out that Kashgari's DëwÃÂn LughÃÂt al-Turk distinguished the Qay tribe from the Qayñàbranch/sub-tribe of the Oghuz-Turkmen tribe.
According to Ottoman tradition, Osman I, the founder of the Ottoman Empire, was a descendant of the Kayñ. This claim has, however, been called into serious question by many modern historians. The only evidence for the Ottomans' Kayñ descent came from genealogies written during the fifteenth century, several centuries after the life of Osman. More significantly, the earliest genealogies written by the Ottomans did not include any reference to Kayñ descent, indicating that it may have been fabricated at a later date.
The famous Oghuz folk narrator, soothsayer and bard Dede Korkut was a Kayñ. In the 10th century, the Central Asian Oghuz Yabgu State was headed by supreme leaders (or Yabghu) who belonged to the Kayñ tribe.
According to Soviet archaeologist and ethnographer Sergey Tolstov, part of the Kayi tribe moved in the Middle Ages from Central Asia to modern day Ukraine, they are known in the Rus' chronicles as kovuy and kaepichi as one of the tribes that formed the Turkic tribal confederation called the Black Klobuks, who were allies of the Rurikids of the Rus' Khaganate; Golden however considers the Kaepichi to be descendants of the para-Mongolic Qay instead.
Soviet and Russian linguist and Turkologist A. V. Superanskaya associates the Kayñ tribe with the origin of the name of the city of Kyiv; however, Canadian Ukrainian linguist Jaroslav Rudnyckyj connects the name Kyiv to the Proto-Slavic root , which should be interpreted as meaning 'stick, pole' as in its modern Ukrainian equivalent ; therefore, the toponym should in that case be interpreted as 'palisaded settlement'.
In Anatolia, twenty seven villages bear the name of .
In Turkmenistan, the tribe is one of the main divisions of the Gökleà  Turkmens living in the Balkan velayat and consists of the following clans: and others. The are also a subtribe of the Bayat Turkmens of the Lebap velayat.
The name and logo of the ðyi Party (ðyi means Good in Turkish) of Meral Akà Âener is inspired by the seal of the Kayñ tribe.