Bretislav I (Czech: Břetislav I.; 1002/1005 – 10 January 1055), known as the "Bohemian Achilles", of the Přemyslid dynasty, was Duke of Bohemia from 1034 until his death in 1055.

In 1039, he invaded Lesser and Greater Poland,[3] captured Poznań,[3] sacked Gniezno, and brought the relics of St. Adalbert,[3] Radim Gaudentius and the Five Brothers back with him.

[3] His main goal was to set up an archbishopric in Prague and create a large state subject only to the Holy Roman Empire.

In 1040, the German King Henry III invaded Bohemia, but was forced to retreat after he lost the Battle at Brůdek (a pass in the Bohemian Forest).

Forced by a mutiny among his nobles and betrayed by Bishop Šebíř of Prague, Bretislav had to renounce all of his conquests save for Moravia and recognize Henry III as his sovereign.

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.

Bretislav died at Chrudim in 1055 during preparations for another invasion of Hungary and was succeeded by his son Spytihněv II as Duke of Bohemia.