Bey, also spelled as Baig, Bayg, Beigh, Beig, Bek, Baeg, Begh, or Beg, is a Turkic title for a chieftain, and a royal, aristocratic title traditionally applied to people with special lineages to the leaders or rulers of variously sized areas in the numerous Turkic kingdoms, emirates, sultanates and empires in Central Asia, South Asia, Southeast Europe, and the Middle East, such as the Ottomans, Timurids or the various khanates and emirates in Central Asia and the Eurasian Steppe. The feminine equivalent title was begum. The regions or provinces where "beys" ruled or which they administered were called beylik, roughly meaning "governorate" or "region" (the equivalent of a county, duchy, grand duchy or principality in Europe, depending on the size and importance of the beylik). However the exact scope of power handed to the beys varied with each country, thus there was no clear-cut system, rigidly applied to all countries defining all the possible power and prestige that came along with the title.

Today, the word is still used formally as a social title for men, similar to the way the titles "sir" and "mister" are used in the English language. Additionally, it is widely used in the naming customs of Central Asia, namely in countries such as Uzbekistan, Kazakhstan and Kyrgyzstan. Notably, the ethnic designation of Uzbeks comes from the name of Öz Beg Khan of the Golden Horde, being an example of the usage of this word in personal names and even names of whole ethnic groups. The general rule is that the honorific is used with first names and not with surnames or last names.

The word entered English from Turkish bey,. Its Old Turkic cognate beg, which – in the form bäg – has been mentioned as early as in the 8th century AD Orkhon inscriptions and is usually translated as "tribal leader". The actual origin of the word is still disputed, though it is mostly agreed that it was a loan-word, in Old Turkic. This Turkic word is usually considered a borrowing from an Iranian language. However, German Turkologist Gerhard Doerfer assessed the derivation from Iranian as superficially attractive but quite uncertain, and pointed out the possibility that the word may be genuinely Turkic. Two principal etymologies have been proposed by scholars:

It was also used by the Uyghurs. It permitted the Turkic Begs in the Altishahr region to maintain their previous status, and they administered the area for the Qing as officials. High-ranking Begs were allowed to call themselves Begs.

Lucy Mary Jane Garnett wrote in the 1904 work Turkish Life in Town and Country that "distinguished persons and their sons" as well as "high government officials" could become bey, which was one of two "merely conventional designations as indefinite as our 'Esquire' has come to be [in the United Kingdom]".

The Republican Turkish authorities abolished the title circa the 1930s.

However, it is important to note that the title Bey, Baig or Begum, Begzada and Uç Bey are regarded as comparable to the European nobility with the title of Viscount, while Sancak-beys and Atabegs are considered to be of an equivalent rank to earls or counts in the context of European nobility.

The title bey (Arabic: بيه Egyptian Arabic pronunciation: [beː]) was also called beyk or bek (بيك) – from Turkish beyg (بيـگ) – in North Africa, including Egypt. A bey could maintain a similar office within Arab states that broke away from the High Porte, such as Egypt and Sudan under the Muhammad Ali Dynasty, where it was a rank below pasha (maintained in two rank classes after 1922), and a title of courtesy for a pasha's son.