OBJECTIVE: To evaluate the impact of a care bundle (antenatal information to women, manual perineal protection and mediolateral episiotomy when indicated) on obstetric anal sphincter injury (OASI) rates. DESIGN: Multicentre stepped-wedge cluster design. SETTING: Sixteen maternity units located in four regions across England, Scotland and Wales. POPULATION: Women with singleton live births between October 2016 and March 2018. METHODS: Stepwise region by region roll-out every 3 months starting January 2017. The four maternity units in a region started at the same time. Multi-level logistic regression was used to estimate the impact of the care bundle, adjusting for time trend and case-mix factors (age, ethnicity, body mass index, parity, birthweight and mode of birth). MAIN OUTCOME MEASURES: Obstetric anal sphincter injury in singleton live vaginal births. RESULTS: A total of 55 060 singleton live vaginal births were included (79% spontaneous and 21% operative). Median maternal age was 30 years (interquartile range 26-34 years) and 46% of women were primiparous. The OASI rate decreased from 3.3% before to 3.0% after care bundle implementation (adjusted odds ratio 0.80, 95% CI 0.65-0.98, P = 0.03). There was no evidence that the effect of the care bundle differed according to parity (P = 0.77) or mode of birth (P = 0.31). There were no significant changes in caesarean section (P = 0.19) or episiotomy rates (P = 0.16) during the study period. CONCLUSIONS: The implementation of this care bundle reduced OASI rates without affecting caesarean section rates or episiotomy use. These findings demonstrate its potential for reducing perineal trauma during childbirth. TWEETABLE ABSTRACT: OASI Care Bundle reduced severe perineal tear rates without affecting caesarean section rates or episiotomy use.