Wenceslaus I (Czech: Václav [ˈvaːtslaf] (listen); c. 911 – 28 September 935 or 929), Wenceslas I or Václav the Good was the duke (kníže) of Bohemia from 921 until his death probably either in 935 or 929 (although 935 is favored today). His younger brother, Boleslaus the Cruel, is commonly considered the perpetrator of Wenceslaus' assassination by the Czech public and the Roman Catholic Church. However, there is a debate on whether his death was an accident or a murder. According to some historians (e. g. Dušan Třeštík or Martin Wihoda) it was the result of a quarrel between the two brothers or their parties (probably after a night of drinking). Boleslaus' men then killed Wenceslaus in the subsequent chaotic fight.

His martyrdom and the popularity of several biographies gave rise to a reputation for heroic virtue that resulted in his elevation to sainthood. He was posthumously declared to be a king and came to be seen as the patron saint of the Czech state. He is the subject of the well-known "Good King Wenceslas", a carol for Saint Stephen's Day.