Enzymatic depolymerization of seaweed polysaccharides is gaining interest for the production of functional oligosaccharides and fermentable sugars. We describe a thermostable alginate lyase belonging to Polysaccharide Lyase family 7 (PL7)， which can be used to degrade brown seaweed， Saccharina latissima， at conditions also suitable for a commercial cellulase cocktail (Cellic CTec2). This enzyme， AMOR_PL7A， is a β-d-mannuronate specific (EC 4.2.2.3) endoacting alginate lyase， which degrades alginate and poly mannuronate within a broad range of pH， temperature and salinity. At 65 °C and pH 6.0， its Km and k values for sodium alginate are 0.51 ± 0.09 mg/mL and 7.8 ± 0.3 s respectively. Degradation of seaweed with blends of Cellic CTec2 and AMOR_PL7A at 55 °C in seawater showed that the lyase efficiently reduces viscosity and increases glucose solublization. Thus， AMOR_PL7A may be useful in development of efficient protocols for enzymatic seaweed processing.