Abstract: The first finds of B–Tm volcanic ash of the catastrophic eruption of Baitoushan Volcano in the 10th century (946/947 AD) in the terrestrial deposits of Primorye are reported. The ash layer is found in 17 cross sections of lacustrine–boggy sediments at nine observation sites, both on the coast and in the internal areas of Sikhote-Alin. Identification of the volcanic source is carried out based on the study of macroelement composition of volcanic glass. Radiocarbon dating is used for age determination. The tephra distribution and the changes in the grain size depending on the distance from the source are analyzed. A comparative analysis of the volcanic glass composition and the data on pyroclastics of the proximal zone and the results of distal tephra study has been performed. It proved that the ash layer in Primorye was formed in the final phase of the eruption, but an exception was the coast of Kit Bay and the Shkotovskoe Plateau, where glass of rhyolitic composition corresponding to the maximum phase at the beginning of the eruption is found. For the reconstruction of the environmental development, the ash is used as an age marker of the Medieval Warm Period in the Holocene, which makes it possible to compare reliably the data on the cross sections located on the coast and in the mountains, to refine the chronology of paleogeographical events of historical time, and to perform interregional correlations.