Bolesław III Wrymouth (Polish: Bolesław III Krzywousty; 20 August 1086 – 28 October 1138), also known as Boleslaus the Wry-mouthed, was the duke of Lesser Poland, Silesia and Sandomierz between 1102 and 1107 and over the whole of Poland between 1107 and 1138. He was the only child of Duke Władysław I Herman and his first wife, Judith of Bohemia.

Bolesław began to rule in the last decade of the 11th century, when the central government in Poland was significantly weakened. Władysław I Herman fell under the political dependence of the Count palatine Sieciech, who became the de facto ruler of the country. Backed by their father, Boleslaw and his half-brother Zbigniew finally expelled Sieciech from the country in 1101, after several years of fighting. After the death of Władysław I Herman in 1102, two independent states were created, ruled by Bolesław and Zbigniew.

Bolesław sought to gain Pomerania which caused an armed conflict between the brothers, and forced Zbigniew to flee the country and seek military help from King Henry V of Germany. Bolesław punished Zbigniew by blinding him. This action caused outrage among supporters of Zbigniew, resulting in a political crisis in Poland. Bolesław once again gained the favor of his subjects with public penance, and made a pilgrimage to the monastery of his patron, Saint Giles, in Hungary.

Bolesław, like Bolesław II the Generous, based his foreign policy on maintaining good relations with neighboring Hungary and Kievan Rus, with whom he forged strong links through marriage and military cooperation in order to break Poland's political dependence on Germany and Henry's vassal, the King of Bohemia, who in moments of weakness of Polish policy was forced to pay tribute in Silesia. These alliances had allowed Bolesław to effectively defend the country from invasion in 1109. Several years later, Bolesław skillfully took advantage of the dynastic disputes in Bohemia to ensure peace on the south-west border.

Bolesław devoted the second half of his rule to the conquest of Pomerania. In 1113 he conquered the northern cities along Noteć, which strengthened the border with the Pomeranians. In subsequent years, he took further steps toward the conquest of Pomerania. The resolution of the conflict with the Holy Roman Empire allowed Bolesław to subordinate Western Pomerania and incorporate Gdańsk Pomerania. The expeditions, carried out in three stages, ended in the 1120s with military and political successes. Integration of the newly annexed lands enabled Bolesław to build churches and begin the process of converting Pomerania. Bishop Otto of Bamberg confirmed the Christianization of Pomerania from 1123 onward.

In the 1130s Bolesław participated in the dynastic dispute in Hungary. After an unexpected defeat, he was forced to make an agreement with Germany. The Congress of Merseburg of 1135 addressed the issues of Pomerania, Silesian (probably also Polish) sovereignty and the supremacy of the Archbishopric of Magdeburg over the Polish Church.

Bolesław was married twice. His first marriage, to the Kievan princess Zbyslava, gave him an excuse to intervene militarily in the internal affairs of Rus'. After her death, Bolesław married a German noblewoman, Salomea of Berg, which in some way was the cause of changes in Polish foreign policy: in the second half of his rule, the Prince sought to restore diplomatic relations with his western neighbor. His last, and perhaps most momentous act, was his will and testament known as "The Succession Statute" in which he divided the country among his sons, leading to almost 200 years of feudal fragmentation of the Polish Kingdom.

Bolesław III Wrymouth has been recognized by historiography as a symbol of Polish political aspirations until well into the 19th century. He also upheld the independence of the Polish archbishopric of Gniezno, despite a temporary failure in the 1130s. Although he achieved undoubted successes, he committed serious political errors, most notably against Zbigniew of Poland, his half-brother. The crime against Zbigniew and his penance for it show Bolesław's great ambition as well as his ability to find political compromise.