In the eastern Adriatic, there are only a few studies on benthic diatoms mostly focused on ecology and taxonomy of periphytic diatoms in the Croatian estuaries or Albanian coastal wetlands. Until now, both planktonic and benthic marine diatoms have never been studied in Bosnia and Herzegovina. This study forms part of a research program investigating the biodiversity and ecosystem functioning in the border marine coastal waters between the two countries – Croatia and Bosnia and Herzegovina. The program includes phycological, zoological and hydrographical research. This paper considers the taxonomic composition and seasonality of marine benthic diatoms in the relation to some key environmental variables (temperature, salinity and density) in the Bay of Neum in Bosnia and Herzegovina on the south-eastern Adriatic coast.