Understanding mental health and substance use treatment seeking and suicidality among suicide decedents is important to determine prevention efforts. To evaluate differences in treatment seeking and suicidality between suicide decedents who died by firearms and those who died by other methods. Cross-sectional data were collected on 234 652 suicide decedents from 2003 to 2018. Participant information was reported by their state of residence to the National Violent Death Reporting System. Statistical analysis was performed from July 1, 2021, to January 21, 2022. Main outcomes were treatment for mental health and substance use at time of death, previous treatment for mental health and substance use, history of suicidal ideation or plans, history of suicide attempts, and disclosure of suicidal ideation or plans. A total of 234 652 participants (182 520 male [77.8%]; 205 966 White [87.8%]; mean [SD] age, 46.3 [18.2] years [range, 3-112 years]) were included in this study. Compared with suicide decedents who died by another method (n = 117 526 [50.1%]), those who died by firearm (n = 117 126 [49.9%]) were more likely to have disclosed thoughts or plans of suicide within the month prior to death (odds ratio [OR], 1.16 [95% CI, 1.13-1.18]) and were less likely to have previously attempted suicide (OR, 0.44 [95% CI, 0.43-0.46]). Compared with those who died by poisoning, those who used a firearm were more likely to have had a history of suicidal thoughts or plans (OR, 1.19 [95% CI, 1.15-1.23]) and to have disclosed their thoughts or plans of suicide within the month prior to death (OR, 1.06 [95% CI, 1.03-1.10]). Compared with those who died by hanging, those who used a firearm were more likely to have disclosed their thoughts or plans of suicide to another person within the month prior to their death (OR, 1.14 [95% CI, 1.11-1.17]). These findings provide information that suggests who is at risk to die by firearm suicide. Community-based interventions in suicide prevention could help reduce access to firearms during a time of crisis. The finding that firearm suicide decedents were more likely to disclose their suicidal thoughts or plans provides an important avenue for prevention.