According to the generally accepted view there were no stainedglass windows in ancient Russian religious architecture. This opinion is undoubtedly based on the absence of stained-glass sets in the then-existing temples that have survived to this day. The information collected by the author of the article, on the contrary, testifies to the use of stained-glass windows in the architecture of Ancient Rus. One of such testimonies is the discovery of three stained-glass windows in the Yaroslavl’ Kremlin, in the cultural layers of the first half of the 13th century. To establish the place of production, the windows were studied using optical emission spectrography. The analysis in all three cases showed the same chemical composition — wood-ash lead glass. This type of glass was imported to Rus’, and is well known in stained glass windows of the twelfth–fifteenth centuries proceeding from what is now Germany and Austria. An additional argument for the non-local origin of the Yaroslavl’ finds is the use of casting and rolling techniques in the manufacture of glass. The present study makes it possible to expand the geography of the distribution of glass with the indicated composition, which, until recently, was mainly limited to the area of Central Europe. The uniformity of all the characteristics of the stained-glass finds indicates that they belong to a single set of stained-glass windows. Most likely, they were used in the Assumption Cathedral, the only stone building that existed in Yaroslavl’ in the pre-Mongolian period. It was founded, according to chronicles, in 1215 by the Rostov prince Konstantin Vsevolodovich. The question of how stained-glass windows from German-speaking countries ended up in Yaroslavl’ was resolved owing to the art history research by V.V. Sedov, who established that the Assumption Cathedral in the capital of the principality, Rostov, was founded by the same prince Konstantin Vsevolodovich in 1213 and built with the participation of Romanesque — apparently German — craftsmen who worked on creating the decor of the building. This suggests that the German craft smen also took part in the decoration of the Yaroslavl’ Cathedral, and this was why the material for future stained-glass compositions was brought to Rus’..