Son of Duke Bogislaw II and Miroslawa, daughter of Duke Mestwin I of Pomerelia, he succeeded to the Duchy of Pomerania-Stettin upon his father's death in 1220; he had however to share the rule of Pomerania with his cousin Wartislaw III, who resided at Demmin. Because he was minor when his father died, until about 1226 his lands were under the regency of his mother Miroslawa from the Pomerelian Samborides dynasty. At first still a Danish fief, the Pomeranian lands fell back to the Holy Roman Empire after the victory of several North-German princes at the 1227 Battle of Bornhöved. Emperor Frederick II of Hohenstufen in 1231 put the Duchy of Pomerania under the suzerainty of the Ascanian margraves of Brandenburg, disregarding the tenure of the Griffin dynasty, and thereby fueling the long-term Brandenburg–Pomeranian conflict. After his cousin Wartislaw III had formally accepted the Brandenburg overlordship by the 1236 Treaty of Kremmen, Duke Barnim I came to terms with the mighty Ascanian margraves in the 1250 Treaty of Landin: he confessed himself a Brandenburg vassal and had to renounce the Uckermark region, nevertheless he reached the consent, that the fief of his cousin Wartislaw would remain with the Griffin dynasty upon his death. When Duke Wartislaw III died in 1264, Barnim I was able to unite the whole Duchy of Pomerania under his rule. He promoted the Ostsiedlung by introducing German settlers and customs into the duchy, established many towns, among them Prenzlau, Szczecin, Gartz, Anklam, Stargard, Gryfino, Police, Pyrzyce, Ueckermünde and Goleniów. He was also known for his generous ecclesiastical foundations, and supported the extension of the secular reign of the Cammin bishops in the Kołobrzeg area. Duke Barnim died at the town of Dąbie, today part of Szczecin. The Minnesinger Meister Rumelant wrote a dirge in his honour.
Marriages and issue
Between 4 September 1238 and 18 July 1242, Barnim I married firstly with Marianna, whose parentage is disputed: she was either identified as a daughter of King Eric X of Sweden, or a member of the House of Ascania as daughter of either Count Albert II of Weimar-Orlamünde, Count Henry I of Anhalt or Albert I, Duke of Saxony. Modern historiography considers that her origins are unknown. They had one daughter:
Between 1253 and 1254, Barnim I married secondly with Margareta, probably a daughter of Nicholas I, Lord of Werle and member of the House of Mecklenburg, although other sources identified her as a daughter of Otto the Child, Duke of Brunswick-Lüneburg. Historian Robert Klempin identified her as the widow of Vitslav I, Prince of Rügen, but this seems very doubtful from a chronological view. They had one son:
Bogislaw IV, co-ruler from 1276 and sole ruler after his father's death, sharing power with his younger half-brothers.
