Phytochemical investigation of Osyris alba L. (Santalaceae) of Jordanian origin resulted in the isolation and identification of one new pyrrolizidine alkaloid, osyrisine (1), together with 16 other known compounds. The structures of all compounds were established on the basis of spectroscopic analysis. Osyrisine, catechin, and catechin-3-O--l-rhamnopyranoside exhibited a significant level of antiparasitic activity against two parasites, Entamoeba histolytica and Giardia intestinalis.