In this study, we analyzed the relationship between state firearm laws and the incidence and severity (i.e., number of victims) of mass public shootings in the United States during the period 1976 -2018. Hypotheses: We hypothesized that states requiring permits to purchase firearms would have a lower incidence of mass public shootings than states not requiring permits. We also hypothesized that states banning large-capacity ammunition magazines would experience a lower number of victims in mass public shootings that did occur than states without bans. Method: We developed a panel of annual, state-specific data on firearm laws and mass public shooting events and victim counts. We used a generalized estimating equations logistic regression to examine the relationship between eight state firearm laws and the likelihood of a mass public shooting. We then used a zero-inflated negative binomial model to assess the relationship between these laws and the number of fatalities and nonfatal injuries in these incidents. Results: State laws requiring a permit to purchase a firearm were associated with 60% lower odds of a mass public shooting occurring (95% confidence interval [CI: Ϫ32%, Ϫ76%]). Large-capacity magazine bans were associated with 38% fewer fatalities (95% CI [Ϫ12%, Ϫ57%]) and 77% fewer nonfatal injuries (95% CI [Ϫ43%, Ϫ91%]) when a mass shooting occurred. Conclusion: Laws requiring permits to purchase a gun are associated with a lower incidence of mass public shootings, and bans on large capacity magazines are associated with fewer fatalities and nonfatal injuries when such events do occur. Public Significance StatementWe cannot definitively conclude that implementing a specific law would lead to a change in the incidence or severity of mass public shootings. However, laws that limit potential shooters' access to firearms by requiring permits may reduce the incidence of mass shootings, and laws that limit the number of shots that can be fired before reloading may reduce the severity of mass public shootings when they do occur. Such laws must be balanced with citizens' right to bear arms under the Second Amendment of the U.S. Constitution.