Alginate and its derivatives are annually produced approximately 30,000 tons or more and are applied to various industries as they are natural polymers. The global market for alginate and its derivatives has been growing steadily. There is little research compared to other enzymes produced through biomass degradation or modification. An alginate lyase, MtAl138, from Microbulbifer thermotolerans DAU221 was cloned and identified in Escherichia coli BL21 (DE3). MtAl138 contains a highly conserved motif (R538TELR, Q607IH609, and YFKAGVY716NQ), which indicates that it belongs to the polysaccharide lyase family 7 (PL7). MtAl138, with a molecular weight of 77 kDa worked optimally at 45 °C and pH 7.4. MtAl138 showed twice as much activity as when there was no NaCl when there was between 100 and 600 mM NaCl. Moreover, its activity increased in organic solvents such as benzene, hexane, methanol, and toluene. Based on the thin layer chromatography analyses, MtAl38 is an endo-type enzyme that produces di-, tri-, or tetrasaccharides from polyG and polyM. This study provided that MtAl138 is an endoenzyme that showed outstanding enzymatic activity at concentrated salt solutions and organic solvents, which makes it a reasonably attractive enzyme for use in various industries.