This study will evaluate the efficacy and safety of plozasiran in approximately 288 adult participants with severe hypertriglyceridemia (SHTG) and history of at least two prior acute pancreatitis (AP) events not attributed to other etiologies, with at least one occurring within the last 12 months prior to screening. Eligible participants will be randomly assigned in a double-blind manner to either receive plozasiran 25 mg by subcutaneous (SC) injection every three months (Q3M) or matching placebo. Enrolled participants will be counseled to remain on the specified low-fat diet and background medications throughout the study. Following completion of the double-blind treatment period, or if the participant has a positively adjudicated AP event (whichever occurs first), participants will transition to the 12-month Open-Label Extension (OLE) treatment period receiving plozasiran 25 mg by SC injection Q3M.