A β-1,3-glucan binding protein (βGBP) specific for laminarin (a β-1,3-glucan) was detected for the first time in a mollusc, Perna viridis. βGBP was isolated and purified from the plasma using laminarin precipitation and affinity chromatography on laminarin-Sepharose 6B, respectively. It agglutinated bakers yeast, bacteria, and erythrocytes and enhanced prophenoloxidase (proPO) activity of the plasma in a dose-dependent manner. The purified βGBP appeared as a single band in native-PAGE and the purity was conformed by HPLC. The protein has a molecular weight estimate of 510 kDa as determined by SDS-PAGE and in isoelectric focusing the purified βGBP was focused as a single band at pI 5.3. βGBP was found to possess inherent serine protease activity but lacked β-1,3-glucanase activity and all these results suggest that plasma βGBP of P. viridis functions as a recognition molecule for β-1,3-glucan on the surface of microbial cell walls. This recognition and binding lead to the activation of the prophenoloxidase cascade mediated by the inherent serine protease activity of βGBP. Presence of agglutinating activity and serine protease activity shows that βGBP is a bifunctional protein. The findings are discussed in light of the importance of this protein in the innate immune response of P. viridis, and they implicate evolutionary link with similar proteins found in other invertebrates.