The filamentous fungus Trichoderma sp. GL2 produces an extracellular glucuronan lyase (GL) when grown on glucuronan as the sole carbon source. In this paper, we report the purification to electrophoretical homogeneity of this polysaccharide lyase by size exclusion chromatography and anion exchange chromatography. The purified GL, classified as an endopolyglucuronate lyase, is a monomer with an apparent molecular weight of 27 kDa and an isoelectric point of 6.95. Despite an inhibition of the activity when polysaccharide substrates were substituted by acetates, the enzyme was active toward glucuronans (acetylated or not) and ulvan, leading to various (4,5)-unsaturated products as oligoglucuronans (acetylated or deacetylated), highly acetylated low-molecular-weight (LMW) glucuronans, and LMW ulvans.