Wenceslaus I (Czech: Václav [ˈvaːtslaf] ( listen); c. 907 – September 28, 935), or Wenceslas I, was the duke (kníže) of Bohemia from 921 until his assassination in 935, purportedly in a plot by his own brother, Boleslav the Cruel.

His martyrdom, and the popularity of several biographies, quickly gave rise to a cult, the Catholic term for a group devoted to someone as a holy person, and a reputation for heroic goodness, resulting in his being elevated to Sainthood, posthumously declared king, and seen as the patron saint of the Czech state. He is even the subject of a Saint Stephen's Day (celebrated on December 26 in the West) Carol, written in 1853 that remains popular to this day, Good King Wenceslas.