A knyaz, also knez, knjaz or kniaz (Church Slavonic: кънѧѕь, romanized: kŭnędzĭ), is a historical Slavic title, used both as a royal and noble title in different times. It is usually translated into English as 'prince', 'king' or 'duke', depending on specific historical context and the potentially known Latin equivalents at the time; the word was originally derived from the common Germanic *kuningaz ('king').

Feminine forms of the word may be divided into two groups:

The title is pronounced and written similarly in different European languages. In Serbo-Croatian and some West Slavic languages, the word has later come to denote "lord", and in Czech, Polish and Slovak also came to mean "priest" (kněz, ksiądz, kňaz) as well as "prince/duke" (knez, kníže, książę, knieža). In Sorbian it means simply "Mister" (from "Master". Compare French monsieur from mon sieur "my lord"), and the Catholic title "monsignor" for a priest. Today the term knez is still used as the most common translation of "prince" in Slovenian, Bosnian, Croatian and Serbian literature. Knez is also found as a surname in former Yugoslavia.

The word is ultimately a cognate of the English King, the German König, and the Swedish Konung. The proto-Slavic form was *kъnędzь, kŭnędzĭ; Church Slavonic: кънѧѕь, kŭnędzĭ; Bulgarian: княз, knyaz; Old East Slavic: князь, knyazĭ; Polish: książę; Serbo-Croatian Latin: knez / Serbo-Croatian Cyrillic: кнез; Czech: kníže; Slovak: knieža; etc. It is generally considered to be an early borrowing from Proto-Germanic kuningaz, a form also borrowed by Finnish and Estonian (kuningas).

The rulers of the Duchy of Poland bore the title of książę, which was rendered as dux or princeps in Latin, and later adopted krol (from Karl, the name of Charlemagne) and its equivalent rex following Bolesław I's coronation in 1025. Similarly, the ruler of the Duchy of Lithuania, called kunigaikštis (also derived from kuningaz) in Polish, was called magnus dux instead of the Polish word for "king", karalius (also derived from Karl). Medieval German records, however, translated knyaz as koning (king) until at least the 15th century.

The meaning of the term changed over the course of history. Initially the term was used to denote the chieftain of a Slavic tribe. Later, with the development of feudal statehood, it became the title of a ruler of a state, and among East Slavs (Russian: княжество (knyazhestvo), Ukrainian: князівство, romanized: kniazivstvo) traditionally translated as duchy or principality, for example, of Kievan Rus'.

In First Bulgarian Empire, Boris I of Bulgaria (852–889) changed his title to knyaz after his conversion to Christianity in 864, abandoning the pagan title 'khan' of his predecessors. The new titles were applied to his sons Vladimir Rasate (889-893) and Simeon I (893–927), however knyaz Simeon took the higher title of tsar soon in 913.

According to Florin Curta, the primary sources have a variety of names for the rulers of the Bulgars before christianisation - such as including ‘rex’, ‘basileus’ and ‘khagan’. Omurtag (814–831) and his son Malamir (831–836) are mentioned in inscriptions as 'kanasubigi'. However, secondary sources are almost always 'khan'.