Trpimir I was a duke in Croatia from around 845 until his death in 864. He is considered the founder of the Trpimirović dynasty that ruled in Croatia, with interruptions, from around 845 until 1091. Although he was formally vassal of the Frankish Emperor Lothair I, Trpimir used Frankish-Byzantine conflicts to rule on his own.

The Duchy of Croatia was a medieval state that was established by White Croats who migrated into the area of the former Roman province of Dalmatia c. 7th century CE. Throughout its existence the Duchy had several seats – namely, Klis, Solin, Knin, Bijaći and Nin. It comprised the littoral – the coastal part of today's Croatia – except Istria, and included a large part of the mountainous hinterland as well. The Duchy was in the center of competition between the Carolingian Empire and the Byzantine Empire for rule over the area. Croatian rivalry with Venice emerged in the first decades of the 9th century and would continue through the following centuries. Croatia also waged battles with the Bulgarian Empire and with the Arabs; it also sought to extend its control over important coastal cities under the rule of Byzantium. Croatia experienced periods of vassalage to the Franks or to the Byzantines and of de facto independence until 879, when Duke Branimir was recognized as an independent ruler by Pope John VIII. The Duchy was ruled by the Trpimirović and Domagojević dynasties from 845 to 1091. Around 925, during the rule of Tomislav, Croatia became a kingdom.