Abstract The marine environment, contains plentiful renewable resources, e.g. macroalgae with unique polysaccharides, motivating search for enzymes from marine microorganisms to explore conversion possibilities of the polysaccharides. In this study, the first GH17 glucanosyltransglycosylase, MlGH17B, from a marine bacterium (Muricauda lutaonensis), was characterized. The enzyme was moderately thermostable with Tm at 64.4 °C and 73.2 °C, but an activity optimum at 20 °C, indicating temperature sensitive active site interactions. MlGH17B uses β-1,3 laminari-oligosaccharides with a degree of polymerization (DP) of 4 or higher as donors. Two glucose moieties (bound in the aglycone +1 and +2 subsites) are cleaved off from the reducing end of the donor while the remaining part (bound in the glycone subsites) is transferred to an incoming β-1,3 glucan acceptor, making a β-1,6-linkage, thereby synthesizing branched or kinked oligosaccharides. Synthesized oligosaccharides up to DP26 were detected by mass spectrometry analysis, showing that repeated transfer reactions occurred, resulting in several β-1,6-linked branches. The modeled structure revealed an active site comprising five subsites: three glycone (−3, −2 and −1) and two aglycone (+1 and +2) subsites, with significant conservation of substrate interactions compared to the only crystallized 1,3-β-glucanosyltransferase from GH17 (RmBgt17A from the compost thriving fungus Rhizomucor miehei), suggesting a common catalytic mechanism, despite different phylogenetic origin, growth environment, and natural substrate. Both enzymes lacked the subdomain extending the aglycone subsites, found in GH17 endo-β-glucanases from plants, but this extension was also missing in bacterial endoglucanases (modeled here), showing that this feature does not distinguish transglycosylation from hydrolysis, but may rather relate to phylogeny.