Background and objective Many countries and all US states have legislation that mandates how children of certain ages and/or sizes should be restrained in vehicles. The objective of the current systematic review was to describe the associations between legislation and three outcomes: child restraint system use, correct child restraint system use and child passenger injuries/deaths. Methods Included studies were published between 2004 and 2020 and evaluated associations between child passenger safety laws and the outcomes described above. Three literature searches using three search terms (child passenger safety, car seat use, booster seat use) were completed in PubMed and PsycINFO, with the last search occurring in January 2021. Studies are presented based on the outcome(s) they evaluated. The original protocol for this review is registered with PROSPERO (ID: CRD42019149682). Results Eighteen studies from five different countries evaluating a variety of different types of legislation were included. Overall, positive associations between legislation and the three outcomes were reported. However, there were important nuances across studies, including negative associations between booster seat legislation and correct child restraint use. Further, there were also negative associations between various types of legislation and outcomes for populations with less formal education and lower incomes, and for racial and ethnic minorities. Conclusion Overall, child passenger safety legislation appears to be positively associated with child restraint system use, correct child restraint use and child passenger injuries/deaths. However, there is a need to more comprehensively characterise how different types of legislation influence child passenger safety outcomes to promote equitable effects across populations.