Research objectives: To study Russian-Horde relations in the period of formation of Russian principalities’ dependence on the Mongol Empire and the ulus of Jochi – a historical phase connected with a 1246 trip of a Russian prince and the widow of his elder brother to Batu. This elder brother, Prince Andrei, had been executed earlier by the Mongols. The aim of the work is to clarify the causes and consequences of the conflict situation that arose as a result of the refusal of Andrei of Chernihiv’s relatives to permit a marriage “according to the Tatar custom”. Research materials: Russian chronicles and collections of Church law (Just Measure, canonical answers of Metropolitan John II [1080–1089]), the so-called “Books of the Pilot”, princely statutes (Statute of Prince Yaroslav of the Church courts), religious letters, the Lyubetsky Synod, as well as some Latin and Muslim sources. Results and novelty of the research: Based on an analysis of the information contained in Old Russian sources of Church law, chronicles, princely religious letters, the reports of the papal diplomat John of Plano Carpini and accompanying representatives of the Franciscan mission, C. de Bridia and Benedict of Poland, as well as sources containing information about the legal norms and customary law of the Mongols (e.g. the travel accounts of William of Rubruck, Marco Polo, and Ibn Battuta), the author comes to the following conclusions: Batu’s demand for marriage between the widow and younger brother of the previously executed Prince Andrei Mstislavovich of Chernihiv “according to the custom of Tatars” fully corresponded to Mongol customary law. However, it came into sharp contrast with two canonical prohibitions of Russian Church law: the ban on marriages of closely related individuals and marriage without a Christian wedding. Possible practical reasons for the Mongol marriage demand could include the desire of the authorities of the ulus of Jochi to exclude the younger brother of Prince Andrei from the number of contenders to rule the Chernihiv Principality, the desire to test the political loyalty of applicants for the inheritance of the executed prince, and an aim to eliminate the dual rule which was ostensibly established in the Chernihiv Principality after the death of its previous ruler. The refusal of the brother and widow of Andrei of Chernihiv to comply with the Batu’s demand caused a strong reaction of the Horde’s authorities, ending in a ritual of forced marriage. This ritual was accompanied by a series of humiliating processes, one of the likely goals of which was to demonstrate to the Russian ruling elites the priority of Mongol legal norms over the legal norms of states that were politically dependent on the Jochids.