Abstract <p>Underwater welding is an important technological process in the marine industry. However, it results in the formation of nanoparticles and microparticles containing toxic metals (such as Cr, Cu, Pb, and Mn) that pose a threat to marine ecosystems. This study evaluated the effect of particles produced by different underwater welding methods (wet welding and flux-cored wire and coated electrode cutting) on the embryonic development of the sea urchin <i>Strongylocentrotus intermedius</i>. Experiments were conducted using welding suspensions at 1 and 10% concentrations to simulate real contamination conditions. The results revealed a notable reduction in embryo viability at every developmental stage (from the initial divisions to the gastrula stage), particularly at high concentrations. Particles from powdered wire (APL-2) and coated electrodes (ELw, ELc) were found to be the most toxic, which correlated with their increased heavy metal content. Exposure to welding particles was found to result in the following: • increased embryo death (up to 11.5% at the blastula stage in the ELc group vs. 5.93% in the control group); • disruption of cell division and morphological abnormalities; • cumulative toxic effects that increase over time. The findings emphasize the need to develop environmentally safe welding technologies and strengthen pollution monitoring in areas of active underwater activity. The study contributes to our understanding of the mechanisms behind the toxic effects of anthropogenic particles on marine organisms and can serve as a basis for environmental regulations.</p>