Importance Firearm injury research in the US has focused on fatal injuries. The incidence and epidemiologic factors associated with nonfatal firearm injuries are less understood. Objective To evaluate estimates of incidence and trends over time of fatal and nonfatal firearm injuries. Design, Setting, and Participants A cross-sectional, ecologic study was conducted using data throughout the US from 2009 to 2017. Data on fatal injuries from the Centers for Disease Control and Prevention were combined with national data on emergency department visits for nonfatal firearm injury from the Nationwide Emergency Department (ED) sample. Data analysis was conducted from August 2019 to September 2020. Exposures Firearm injuries identified with International Classification of Diseases external cause of injury codes and categorized by intent of injury, age group, and urban-rural location. Main Outcomes and Measures Incidence, case fatality rate, and trends over time of firearm injury according to intent, age group, and urban-rural location. Results From 2009 to 2017, there was a mean of 85 694 ED visits for nonfatal firearm injury and 34 538 deaths each year. An annual mean of 26 445 deaths (76.6%) occurred outside of the hospital. Assault was the most common overall mechanism (38.9%), followed by unintentional injuries (36.9%) and intentional self-harm (19.6%). Self-harm, which accounted for 21 128 deaths (61.2%), had the highest case fatality rate (89.4%; 95% CI, 88.5%-90.4%), followed by assault (25.9%; 95% CI, 23.7%-28.6%) and legal intervention (23.4%; 95% CI, 21.6%-25.5%). Unintentional injuries were the most common nonfatal injuries (43 729 [51.0%]) and had the lowest case fatality rate (1.2%; 95% CI, 1.1%-1.3%). Self-harm deaths, 87.8% of which occurred outside the hospital, increased in all age groups in both rural and urban areas during the study period and were most common among people aged 55 years and older. The rate of fatal assault injuries was higher in urban than in rural areas (16.6 vs 9.0 per 100 000 per year) and highest among people aged 15 to 34 years (38.6 per 100 000 per year). Rates of unintentional injury were higher in rural than in urban areas (18.5 per 100 000 vs 12.4 per 100 000). Conclusions and Relevance In this cross-sectional study, suicide appears to be the most common cause of firearm injury death in the US, and most people who die from suicide never reach the hospital. These findings suggest that assaults and unintentional injuries account for most nonfatal and overall firearm injuries and for most of the injuries that are treated in hospitals. This cross-sectional study examines changes in incidence and trends of fatal and nonfatal firearm injury by intent associated with the injury. Question What is the incidence of fatal and nonfatal firearm injury in the US, including self-harm, assault, and unintentional injury? Findings In this cross-sectional study of data from nationwide US databases, from 2009 to 2017, there was an annual average of 85 694 emergency department visits for nonfatal firearm injury and 34 538 deaths from firearm injury. Although suicide accounted for 61% of deaths, self-harm accounted for only 3% of nonfatal injuries; assaults accounted for 35% of deaths and 41% of nonfatal injuries, and unintentional injuries accounted for 2% of deaths and 51% of nonfatal injuries. Meaning In the US, nonfatal firearm injuries are more than twice as prevalent as deaths from firearm injury; the most common injuries are self-harm among deaths, unintentional injuries among nonfatal injuries, and assaults overall.