This article explores how dynastic centralisation in early seventeenth-century Sweden shaped relations between dukes and kings as well as their respective courts. Focusing on Duke Johan of Östergötland, it examines how courtly socialisation influenced Johan’s loyalty to his uncle, Charles IX, and cousin, Gustavus Adolphus. Drawing on correspondence and court records, the study argues that Johan’s childhood socialisation was instrumental in fostering his allegiance and contributed to a novel court configuration. While ducal courts in sixteenth-century Sweden had often been divisive, the early seventeenth century witnessed the emergence of a more cohesive constellation of courts. The ducal court remained a platform for princely display and patrimonial authority, but the relationship between ducal and royal courts was characterised by a harmonious coexistence and interaction. This transformation was intrinsically linked to dynastic centralisation, which redefined the role of cadet princes, fostering greater compliance within the royal family and hierarchy.