This paper aims to assess the current and potential role of UNESCO Transboundary Biosphere Reserves (TBRs) as a platform for cross-border cooperation enhancing sustainable development of border areas characterised by unique natural and cultural heritage but lagging in socio-economic terms. The study covered three TBRs located in the Polish--Ukrainian borderland: West Polesie, Roztocze and East Carpathians. The quantitative (regarding subject matter, partners involved, amount of funding, etc.) and qualitative analysis (aimed at examining the objectives of the initiatives undertaken in line with the BRs' goals) included projects implemented under the CBC Programmes Poland--Belarus--Ukraine in the years 2004-2020. The results suggest that cross-border cooperation in the analysed areas was generally oriented toward TBR-related objectives, i.e., socio-culturally and ecologically sustainable development. At the same time, numerous barriers have been identified that limit the impact of crossborder cooperation on the sustainable development of the TBRs. [ABSTRACT FROM AUTHOR]