Abstract: Polygonum hydropiper of the family Polygonaceae has been considered as an important plant in the traditional Ayurvedic and in the indigenous medical system in Bangladesh. As membrane stabilizing activity, thrombolytic activity and cytotoxic activity of Polygonum hydropiper were not studied earlier, our aim is to find out those activities of Polygonum hydropiper. In the present study the phytochemical screening, thrombolytic activity, membrane stabilizing activity and cytotoxic activity of Polygonum hydropiper was evaluated. For phytochemical screening, some common standard tests those are available for phytochemical screening was done. Cytotoxicity test was done by Brine shrimp lethality bioassay. The thrombolytic and membrane stabilizing activities were assessed by using human erythrocyte and the results were compared with standard streptokinase (SK) and standard anti-inflammatory drug, Acetyl Salicylic Acid (ASA) or aspirin, respectively. Carbohydrates, steroids, flavonoids and saponins were present in different solvent extracts. The extracts demonstrated significant toxicity to A. salina with LC50 values ranging from 1.59 to 3.39 μg mL-1 and LC90 values ranging from 32.36 to 85.11 μg mL-1 as compared to standard Vincristine sulphate (VS, LC50 value 0.927 μg mL-1 and LC90 value 6.310 μg mL-1). The ethanol soluble fraction of ethanol extract of Polygonum hydropiper revealed highest thrombolytic activity 43.08%. For hypotonic solution induced haemolysis, at a concentration of 1.0 mg mL-1, the Methanolic Extract (ME) inhibited 85.06% haemolysis of RBCs. Our study demonstrates that Polygonum hydropiper possesses significant thrombolytic and membrane stabilizing activity and shows moderate cytotoxic activity.