Polysaccharides from seaweeds are widely used in various fields, including the food, biomedical material, cosmetic, and biofuel industries. Alginate, which is a major polysaccharide in brown algae, and the products of its degradation (oligosaccharides) have been used in stabilizers, thickeners, and gelling agents, especially in the food industry. Discovering novel alginate lyases with unique characteristics for the efficient production of oligosaccharides may be relevant for the food and pharmaceutical fields. In this study, we identified a unique alginate lyase derived from an alginate-utilizing bacterium, Shewanella species YH1. The recombinant enzyme (rAlgSV1-PL7) was produced in an Escherichia coli system and it was classified in the Polysaccharide Lyase family 7. The optimal temperature and pH for rAlgSV1-PL7 activity were around 45 °C and 8, respectively. Interestingly, we observed that rAlgSV1-PL7 retained over 80% of its enzyme activity after incubation at 30 °C for at least 20 days, indicating that rAlgSV1-PL7 is a long-lived enzyme. Moreover, the degradation of alginate by rAlgSV1-PL7 produced one to four sugars because of the broad substrate specificity of this enzyme. Our findings suggest that rAlgSV1-PL7 may represent a new commercially useful enzyme.