Boleslav (or Boleslaus) I the Cruel (? - 972), was the Duke of Bohemia from 929 or 935 to July 15, 972.

His father was Vratislav I of Bohemia, Duke of Bohemia. Boleslav I had a son named Boleslav II the Pious of Bohemia, Duke of Bohemia, and a daughter named Dobrava/Dubrawka of Bohemia.

Boleslav is notorious for the murder of his brother St Wenceslas, the result of which brought him to the Czech (ducal) throne. Nevertheless Boleslav is generally respected by Czech historians as an energetic ruler. Citing Wenceslas' religious policies as the cause of Boleslav's fratricide seems unlikely as Boleslav in no way impeded the growth of Christianity in Bohemia, and in fact he actually sent his sister Mlada (a nun) to Rome to ask permission to make Prague a bishopric.

One major policy shift after the death of Wenceslas was regarding Czech-German relations. It is usually asserted that Wenceslas was an obedient client of the German King Henry the Fowler. Boleslav on the other hand, found himself almost immediately at war with Henry's successor Otto I the Great. This conflict, presumably consisting of border raids between Boleslav on one side and the Margrave of the Ostmark on the other (the general pattern of warfare in this region at the time) reached its conclusion in 950 when Boleslav signed a peace with Otto. It cannot be said for certain if Boleslav became a vassal of the German King, but it is known that he led a Czech force in alliance with Otto at the great victory over the Magyars at the Lech river (August 10, 955). He had also helped Otto to crush an uprising of Slavs on the Lower Elbe in 953.

Czech historians also claim that Boleslav expanded his power into Silesia, Lusatia and Moravia, but no dates are given for these alleged conquests. If they did occur, they must have been only transistory gains because Boleslav's successors had to conquer them all over again. Boleslav realised the growth of Polish strength to the north of his borders and he accordingly arranged for his sister Dobrava to marry the Piast prince Mieszko I in 965.