We purified and characterized a novel protease with fibrinolytic activity from the hemolymph of a polychaeta, Periserrula leucophryna. The enzyme was isolated by chromatographic methods using Phenyl-Sepharose and Benzamidine-Sepharose. SDS-PAGE and gel filtration revealed a single polypeptide chain with a molecular weight of 30 kDa. The N-terminal sequence was determined to be IVGGQNARQGEFPWQV. The purified enzyme preferentially cleaved the synthetic substrates that had Lys (rather than Arg) at the $P_1$ position and did not efficiently cleave substrates with nonpolar amino acids. Among chromogenic protease substrates, the substrate that was most susceptible to hydrolysis by Periserrula leucophryna fibrinolytic protease (PLFP) was Val-Leu-LyspNA (substrate for plasmin). The inhibition profile revealed the protease belongs to a family of serine proteases and has plasmin-like activity that is strongly inhibited by ${alpha}2$-antiplasmin. The purified PLFP was able to dissolve the artificial fibrin, and its fibrinolytic behavior is similar to that of plasmin. In conclusion, PLFP is a novel protease and has potential for practical applications in thrombolytic therapy.