The aim of this work is to identify the provenance of the pre-Romanesque church no. IX (the so-called four-chevet rotunda) in Mikulčice from the 2nd half of the 9th century and determine its probable function. Since the church no. IX is a tetraconch inscribed into the shell of a rotunda, it is necessary to compare it with the tetraconchs from the 4th to 11th century. The tetraconchs occur in this period mainly in the area of the Byzantine architecture and in the areas influenced by the Byzantine architecture. The ground plan of the church no. IX resembles most closely some Armenian tetraconchs inscribed into a rotunda from the 7th to the 11th century. The Byzantine mission to Moravia from 863 can be identified as the most likely mediator of transmission of the idea of the tetraconch inscribed into a rotunda from the eastern Black-Sea area into the Great-Moravian environment. According to the archaeological context, the church no. IX fulfilled only two specific functions – the function of the cemetery chapel and the function of the baptistery.