Trpimir I, Prince (Slavic: Knez) of Dalmatia (TransSavian Croatia) in 845–864 and the Duke of Croats since 852. He is the founder of the Croatian House of Trpimir. Although he was a faithful vassal of the Frankish Emperor Lothair I, Trpimir used the mettling Frankish-Byzantine conflicts and rule on his own.
Trpimir succeeded Dalmatia's Prince Mislav in 845 and asserted his throne in Klis. Trpimir battled successfully against his neighbours, the Byzantine coastal cities under the strategos of Iader in 846–848. In 853 he repulsed an attack from an Army of the Bulgarian Khan Boris I and concluded a peace treaty with him, exchanging gifts.
On 4 March 852 Trpimit issued a Latin charter, confirming Mislav's donations to the Archbishopric in Split. In this document, dated, Trpimir named himself the "by the mercy of God, Duke of Croats" (Latin: Dux Chroatorum iuvatus munere divino) and his realm as the "Realm of the Croats" (Regnum Chroatorum).
The famous theologian Gottschalk of Orbais was at Trpimir's court between 846 and 848, and his work De Trina deitate is an important source for Trpimir's reign. During Trpimir's reign, the first obvious dividations in the Christian Church appeared, further dividing the Croats and Serbs, with the Croats falling under the western Latin-Roman-German sphere of influence. Trpimir founded the House of Trpimir that would rule Croatia until the end of the 11th century.