Importance. Firearm violence is a significant public health and safety problem in the United States. A surge in firearm purchases following the onset of the coronavirus pandemic may increase rates of firearm violence. Objective. To estimate the association between changes in firearm purchasing and interpersonal firearm violence during the coronavirus pandemic. Design. Cross-sectional time series study. We estimate the difference between observed rates of firearm purchases and those predicted by seasonal autoregressive integrated moving average models. Using negative binomial models, we then estimate the association between excess firearm purchases and rates of interpersonal firearm violence within states, controlling for confounders. Setting. The 48 contiguous states and the District of Columbia. Hawaii and Alaska are excluded due to missing or incomplete data. Exposure. The difference between observed and expected rates of firearm purchases in March through May 2020, approximated by National Instant Criminal Background Check System records. Main Outcome and Measure. Fatal and nonfatal injuries from interpersonal firearm violence, recorded in the Gun Violence Archive. Results. We estimate that there were 2.1 million excess firearm purchases from March through May 2020--a 64.3% increase over expected volume, and an increase of 644.4 excess purchases per 100,000 population. We estimate a relative rate of death and injury from firearm violence of 1.015 (95% Confidence Interval (CI): 1.005 to 1.025) for every 100 excess purchases per 100,000, in models that incorporate variation in purchasing across states and control for effects of the pandemic common to all states. This reflects an increase of 776 fatal and nonfatal injuries (95% CI: 216 to 1,335) over the number expected had no increase in purchasing occurred. Conclusions and Relevance. We find a significant increase in firearm violence in the United States associated with the coronavirus pandemic-related surge in firearm purchasing. Our findings are consistent with existing research. Firearm violence prevention strategies may be particularly important during the pandemic.