Objective: To compare pre-pregnancy risk factors associated with severe preeclampsia/eclampsia (SPE) and/or HELLP syndrome among singleton versus twin pregnancies. Design: A population-based retrospective cohort study. Setting: British Columbia (BC), Canada. Population: All pregnancies with singletons or twins that resulted in a stillbirth or live birth at ≥20 weeks’ gestation from 2008/09 to 2020/21. Methods: Data were obtained from the BC Perinatal Database Registry. Logistic regression was used to estimate the association between each risk factor (e.g., body-mass-index (BMI), in-vitro-fertilization (IVF), chronic hypertension, and diabetes) and SPE/HELLP, as well as the modifying effect of plurality. Main Outcome Measures: Severe preeclampsia, eclampsia, and/or HELLP syndrome. Results: Among 563,252 pregnancies (8,841 twin, 554,411 singleton), the rate of SPE/HELLP was 4.7 per 1,000 singleton pregnancies and 31.1 per 1,000 twin pregnancies (relative risk 6.61, 95% confidence interval [CI] 5.84-7.49). Older maternal age (≥35 years), nulliparity, pre-pregnancy and gestational diabetes, chronic hypertension, prior mental health problems, substance use during pregnancy and prior stillbirth increased the odds, while smoking decreased the odds of SPE/HELLP among both singletons and twins. However, the adjusted associations between BMI, IVF, prior abortions and SPE/HELLP differed by plurality: IVF and high BMI were associated with elevated risks in singleton pregnancies but not in twins, while a history of prior abortions was associated with decreased risk in twin but not singleton pregnancies. Conclusions: High BMI and IVF are associated with elevated risk of SPE/HELLP syndrome in singleton pregnancies, but not in twin pregnancies. This study provides insights regarding SPE/HELLP syndrome among singleton and twin pregnancies, and useful information for pre-pregnancy counselling.