This Phase 3 study will evaluate the safety and efficacy of plozasiran injection (ARO-APOC3) in adult participants with severe hypertriglyceridemia (SHTG). After providing informed consent eligible participants will be randomized to receive 4 doses (once every 3 months) of plozasiran or placebo, and be evaluated for efficacy and safety. After month 12, eligible participants will be offered an opportunity to continue in an optional open-label extension under a separate protocol.