The relationship between King Alfonso V of Aragon during his reign in Naples (1442-1458) with the Bosnian Grand Voivode and later Duke Stjepan (Stephen) Vukčić Kosača has been the topic of numerous research papers and monographs, but there are still possibilities for further insights on this matter. Along with Skanderbeg and Serbian Despot Đurađ (George) Branković, Kosača was considered the most prominent ally of the Neapolitan king, although the results of their cooperation were relatively modest at the level of military projects and foreign policy, and significantly greater in terms of commerce and exchanges of cultural models and political symbolism. This paper provides an overview of all aspects of the relationship between Alfonso V and Stjepan Vukčić, with particular emphasis on issues which remained insufficiently clarified or ignored by previous researchers. This includes the development of diplomacy and court culture, Vukčić’s ducal title, marital strategies and economic ties between Bosnia and Apulia. It may be concluded that the partnership was most likely initiated at Kosača’s initiative because both parties shared enmity for Venice, and the Bosnian magnate was under attack from various sides in 1443 and 1444. After ten years, the pact was renewed in 1454 after the fall of Constantinople, when the Ottoman threat became a regional problem. However, no mutual Crusade was ever organised. In the meantime, Alfonso helped Stjepan in mediation with the key players in South East European politics, but also by providing him with mercenaries and, occasionally, with more limited naval support. Kosača refrained from taking part in the war between Naples and Venice (1449-1450), while Alfonso similarly did not come to Stjepan’s aid in his war against Dubrovnik (1451-1454). Both of them acted in such a way to avoid trouble in their own states. Meanwhile, Grand Voivode Stjepan assumed the title of Duke of St. Sava, possibly influenced by his relations with his Neapolitan senior. He and his courtiers, especially those acting as ambassadors, acquired knighthood and membership in Neapolitan (and other) chivalric orders. It is also possible that Duke Stjepan’s second wife, Barbara de Payró, was of Catalan origin, although this is little more than speculation. Commerce was based on the trade in people and goods. The Kingdom of Naples, especially Apulia, could offer grain, salt and wool, but also mercenaries and military equipment, while Bosnia supplied Alfonso’s kingdom with lead and “slaves,” usually used as household servants. The relationship forged by King Alfonso and Duke Stjepan was maintained by their heirs and it ended only in 1480s, when the land of the Kosačas was completely overrun by the Ottomans.