Břetislav (b. between 1005-1012, d. January 10, 1055) of the house of Premyslids was a duke of Bohemia from 1035 till 1055.
Břetislav was a son of duke Oldřich, then the protector of the Žatecko province, and his would-be wife Božena. In 1019 at Schweinfurt he kidnapped his future wife Judith of Schweinfurt (Jitka), a daughter of a Bavarian margrave, Henry of Schweinfurt of Nordgau.
During his father’s reign, in 1029, he took back Poland. About 1031 Břetislav invaded Hungary in order to prevent its expansion under king Stephen. The partition of Bohemia between Oldřich and his brother Jaromir in 1034 was probably the reason why Břetislav fled beyond Bohemian border only to come back to take the throne after Jaromir’s abdication.
In 1035 Břetislav helped Emperor Conrad II in his war against the Lusatians. In 1038 he invaded Little Poland, captured Krakow and Poznan and sacked the capital, Gniezno, bringing the relics of St Adalbert back with him. On the way back he conquered part of Silesia including Wrocław. His main goal was to set up an archbishopric see in Prague and create a large state subject only to the Holy Roman Empire. In 1041 the German King Henry III invaded Bohemia but was forced to retreat by an ambush on his supply lines. However, Bretislav was aware that he could not hold out indefinitely against the Germans and signed a truce with Henry III. In the ensuing peace treaty Bretislav renounced all of his conquests save for Moravia.
In 1047 Emperor Henry III negotiated a peace treaty between Bretislav and the Poles. This pact worked in Bretislav's favour as the Polish ruler swore never again to attack Bohemia in return for an annual subsidy to Gniezno. In 1054 Bretislav issued the famous Seniority Law. For the first time this act stated that Bohemia and Moravia would pass directly through the senior line of the Premyslid dynasty. Younger members of the dynasty were allowed to govern Moravia, but only at the Duke's discretion.
Bretislav was the author of decrees concerning the rules of Christianization, which included a ban on polygamy or trade on holidays.
Břetislav died at Chudrim in 1055 during his preparation for another invasion of Hungary and was succeeded by his son Spythinev II.