Abstract: The U.S. Department of Defense and other stakeholders recommend lethal means safety counseling (LMSC) to reduce suicide risk among military service members. Despite the promise of LMSC, few studies have examined moderators of LMSC treatment outcomes, such as posttraumatic stress disorder (PTSD) symptoms. Individuals with elevated PTSD symptoms are characteristically hypervigilant to threat and are more likely to store their firearms unsafely, which might impact their treatment response to LMSC. In this secondary analysis of the Project Safe Guard LMSC intervention, 209 firearm-owning members of the Mississippi National Guard completed self-report surveys (M [SD] age = 35.2 [10.1] years; 86.6% male, 79.4% White). We used logistic regression to examine the moderating effect of PTSD symptoms (PTSD Checklist for DSM-5; e.g., hyperarousal symptoms) on the association between treatment groups (LMSC vs. control; cable lock provision vs. no cable lock provision) and the use of new locking devices at 6-month follow-up. At 6-month follow-up, 24.9% (n = 52) of participants reported new firearm locking device use. The interaction between hyperarousal symptoms and LMSC (vs. control) was significant. Specifically, LMSC increased the use of new firearm locking devices relative to control at 6-month follow-up for individuals with low/medium, but not high, levels of baseline hyperarousal symptoms. Hyperarousal symptoms did not moderate the association between cable lock provision (vs. no cable lock provision) and use of new locking devices. Findings suggest that existing LMSC interventions need to be adapted for use with service members with elevated hyperarousal symptoms.