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Soil salinity is an enormous problem affecting global agricultural productivity. Deep-sea actinobacteria are interesting due to their salt tolerance mechanisms. In the present study, we aim to determine the ability of deep-sea <i>Dermacoccus</i> (<i>D. barathri</i> MT2.1^T and <i>D. profundi</i> MT2.2^T) to promote tomato seedlings under 150 mM NaCl compared with the terrestrial strain <i>D. nishinomiyaensis</i> DSM20448^T. All strains exhibit in vitro plant growth-promoting traits of indole-3-acetic acid production, phosphate solubilization, and siderophore production. Tomato seedlings inoculated with <i>D. barathri</i> MT2.1^T showed higher growth parameters (shoot and root length, dry weight, and chlorophyll content) than non-inoculated tomato and the terrestrial strain under 150 mM NaCl. In addition, hydrogen peroxide (H₂O₂) in leaves of tomatoes inoculated with deep-sea <i>Dermacoccus</i> was lower than the control seedlings. This observation suggested that deep-sea <i>Dermacoccus</i> mitigated salt stress by reducing oxidative stress caused by hydrogen peroxide. <i>D. barathri</i> MT2.1^T showed no harmful effects on <i>Caenorhabditis elegans</i>, <i>Daphnia magna</i>, <i>Eisenia foetida</i>, and <i>Escherichia coli</i> MC4100 in biosafety tests. This evidence suggests that <i>D. barathri</i> MT2.1^T would be safe for use in the environment. Our results highlight the potential of deep-sea <i>Dermacoccus</i> as a plant growth promoter for tomatoes under salinity stress.