Introduction and hypothesis: Obstetric anal sphincter injuries (OASIS) are associated with an increased risk of faecal incontinence after vaginal delivery. The aim of this retrospective population-based cohort study was to assess whether mediolateral episiotomy is associated with a reduction in the rate of OASIS during operative vaginal delivery. Methods: We used data from the Dutch Perinatal Registry (Perined) that includes records of almost all births between 2000 and 2010 in The Netherlands. In a cohort of 170,969 primiparous and multiparous women whose delivery was recorded, we estimated the association between mediolateral episiotomy and OASIS following both vacuum and forceps deliveries using univariate and multivariate logistic regression analysis. Results: The incidences of OASIS following vacuum delivery in 130,157 primiparous women were 2.5% and 14% in those with and without a mediolateral episiotomy, respectively (adjusted OR 0.14, 95% CI 0.13–0.15), and in 29,183 multiparous women were 2.0% and 7.5%, respectively (adjusted OR 0.23, 95% CI 0.21–0.27). The incidences of OASIS following forceps delivery in 9,855 primiparous women were 3.4% and 26.7% in those with and without a mediolateral episiotomy, respectively (adjusted OR 0.09, 95% CI 0.07–0.11), and in 1,774 multiparous women were 2.6% and 14.2%, respectively (adjusted OR 0.13, 95% CI 0.08–0.22). Conclusions: The use of a mediolateral episiotomy during both vacuum delivery and forceps delivery is associated with a fivefold to tenfold reduction in the rate of OASIS in primiparous and multiparous women.