Background: Despite a vast literature linking in-home firearm access to youth suicides, few studies have examined whether and how "gaining" access to household firearms shapes adolescents' development of suicidal ideation. Objective: This study examined whether gaining access to household firearms among U.S. adolescents is associated with suicidal ideation. To provide insights into the mechanisms underlying this relationship, this study assessed whether the association differs between adolescents with varying degrees of suicidal motives. This study focused on exposure to childhood maltreatment as suicidal motivation of interest. Method: This study used a nationally representative sample of 9973 U.S. adolescents from the National Longitudinal Study of Adolescent to Adult Health (Add Health). To account for a number of selection factors, exploiting the longitudinal nature of data and a rich set of individual- and family-level measures, this study employed OLS regression models with a lagged dependent variable and fixed effects models. Results: Results revealed that gaining access to household firearms is associated with an increased risk of suicidal ideation for both male and female adolescents. This association was significantly greater for adolescents with history of childhood maltreatment among males, but not females. Conclusions: This study supports the hypothesis that in-home firearm access is predictive of male adolescents' suicidal ideation and this association is driven largely by those with history of child maltreatment. In contrast, for females, the association is not conditional on exposure to childhood maltreatment. These results suggest that the pathways through which in-home firearm access shapes adolescent suicidal ideation may be gender-specific.