Poseł moskiewski (‘The Muscovite Envoy’) by Jan Żabczyc is one of many epic poems focusing on political issues in the beginning of the seventeenth century in the Polish- Lithuanian Commonwealth. Discussed by a few philologists, mainly in the twentieth century, the poem was still deserving of a systematic analysis. This study aims to fill this gap and thoroughly investigate the subject of diplomacy in the poem. The author proposes to examine Żabczyc’s work using the methodology known as New Diplomatic History. There are several epic poems in early modern Polish literature exploring the subject of diplomacy (e.g. Przeważna legacyja by Samuel Twardowski or Poselstwo i krótkie spisanie rozprawy z Moskwą by Eliasz Pielgrzymowski), yet few of them illustrate diplomatic actions between allied countries and sovereigns as Żabczyc’s poem does. The article is divided into three main parts. First, the author introduces the subject, genre and historical background of the poem. The main content of the poem is a legation from False Dmitry to Cracow sent in order to keep an alliance with the Commonwealth by wedding per procura of Dmitry and Maryna Mniszech. Similar works of literature from the seventeenth century are also presented. The second part focuses on the details of the poem, referred to as a diplomatic poem, in New Diplomatic History’s language, since the subject of diplomacy determines the structure, the nature of the characters and the main purpose of their actions: agreement and settlement, as opposed to victory over the enemy as the subject of heroic poems. The third part provides a detailed analysis of diplomatic ceremonies, gifts, habits and customs, e.g. speeches and table seating during the wedding. It also proposes an interpretation of the tzar and aristocrat’s wedding as an image of the alliance between Russia and the Commonwealth. It is concluded that Poseł moskiewski is a short but important poem in the history of Polish epics. As a representative of diplomatic literature, it is unique among early modern Polish epic poetry.