Abstract: The growing exploitation of marine space for human activities is leading to the proliferation of artificial struc­ tures in coastal and offshore areas. Hence, improving building materials to minimize the impact of their intro­ duction in the marine environment and to enhance their overall environmental sustainability is essential. The addition of biochar, a common carbon-rich by-product of thermochemical treatments of discarded biomass, in concrete mixtures for marine uses may lead to progresses in this perspective, combining waste recycling with carbon storage and sequestration potential. However, comprehensive evaluations of possible effects of biocharbased concrete on the marine biota are lacking to date. Here, laboratory tests and manipulative field experiments were carried out in three different sites (NE Adriatic, Mediterranean Sea) to assess whether biochar-based concrete could affect marine organisms and the colonization of micro- and macrobenthic assemblages on arti­ ficial hard substrata in shallow subtidal habitats. No relevant ecotoxicological responses to biochar exposure for several target marine organisms from different trophic levels were found. With respect to conventional concrete, adding up to 10 %wt. biochar to the mixture did not determine significant changes neither in microfouling assemblages at early stages of colonization, nor in the structure of macrobenthic assemblages and their total biomass on submerged substrata during the six-month field experiment. Although further investigations in other habitats and with different types of biochar are needed to draw generalizations, such findings highlight that biochar-based concrete can be biologically suitable for marine constructions and could contribute to reduce the carbon footprint of marine artificial structures.