Boleslav (Boleslaus) III (died 1035/1036) was duke of Bohemia from 999 until 1002.

The eldest son of Boleslaus II the Pious. Boleslaus III was a weak ruler in whose chaotic reign, Bohemia became a pawn in the long war between the Holy Roman Emperor Henry II and Boleslaus the Brave, King of Poland. Boleslaus became duke of Bohemia in 999. By 1002 a revolt forced Boleslaus to flee to Poland whilst his kinsman Vladivoj took the Czech throne.

On February 9 1003 Boleslaus was restored to authority with armed support from Heinrich von Schweinhurst, Margrave of the Ostmark. The duke soon undermined his own position by ordering a massacre of his leading nobles at Vysehrad.

Boleslaus's brothers Jaromir and Oldrich fled to Germany and placed themselves under the protection of Henry II. At the same time Boleslaus the Brave, claiming the ducal throne for himself, invaded Bohemia and took Prague without any serious opposition. Boleslaus III was blinded and imprisoned, probably dying in captivity some thirty years later.