Coloman was crowned king of Croatia in Biograd na Moru in 1102. In the 13th century, Thomas the Archdeacon wrote that the union of Croatia and Hungary was the consequence of conquest. However, the late 14th-century Pacta conventa narrates that he was only crowned after he had reached an agreement with twelve leading Croatian noblemen, because the Croats were preparing to defend their kingdom against him by force. Whether this document is a forgery or an authentic source is a subject of scholarly debate.

In an attempt to prevent an alliance between Coloman and Bohemond I of Antioch, the Byzantine Emperor Alexios I Komnenos arranged a marriage between his son and heir, John, and Coloman's cousin, Piroska, in 1104 or 1105. The alliance with the Byzantine Empire also enabled Coloman to invade Dalmatia in 1105.According to the Life of the blessed John of Trogir, he personally commanded his troops besieging Zadar, the most influential among the Dalmatian towns. The siege lasted until Bishop John of Trogir negotiated a treaty between Coloman and the citizens who accepted the king's suzerainty. The town of Split likewise surrendered after a short siege, but two other Dalmatian towns—Trogir and Šibenik—capitulated without resistance. The Life of St Christopher the Martyr also says that a Hungarian fleet subjugated the islands of the Gulf of Kvarner, including Brač, Cres, Krk, and Rab. Thomas the Archdeacon narrates that Coloman granted each Dalmatian town its own "charter of liberties" to secure their loyalty. These liberties included the citizens' right to freely elect the bishop of their town and their exemption from any tribute payable to the monarch. Following his conquest of Dalmatia, Coloman assumed a new title—"King of Hungary, Croatia and Dalmatia"—which was first recorded in 1108.