Distribution and composition of thiotrophic mats were analyzed using high-resolution biogeochemical and community fingerprinting at the outer shelf of the Northwestern Black Sea. Dives with the submersible JAGO showed accumulations of organic matter forming dark patches on the seafloor, which were covered by mat-forming whitish microbial filaments. Where the Black Sea chemocline met the seabed (between 150 - 170 m), these mats covered between 25 to 55 % of the seafloor, and appeared to form a belt of 3 km width. The mats were composed of Beggiatoa-like large filamentous sulfur bacteria based on 16S rRNA sequences from the mat. The microbial community under the mats was enriched with taxa affiliated with polymer degrading, fermenting and sulfate reducing microorganisms. Under the mats, higher organic matter accumulation, as well as higher remineralization and sulfate reduction rates were measured compared to outside the mat. Mat-covered and mat-free sediments showed similar degradability of the bulk organic matter pool, suggesting that the higher sulfide fluxes and subsequent development of the thiotrophic mats in the patches are consequences of the accumulation of organic matter rather than its qualitative composition. Our observations suggest that the key factors for the distribution of thiotrophic mat-forming communities near to the Crimean shelf break are (1) hypoxic conditions repressing grazers and favoring thiotrophic bacteria and (2) the specific seafloor topography that enhances the accumulation of labile organic matter.