Traumatic life events and posttraumatic stress disorder (PTSD) symptomatology are associated with suicidality, including suicidal ideation and behavior. The current study investigates the moderating role of mindfulness facets, as measured by the Five Facet Mindfulness Questionnaire, in the association between PTSD symptom severity and suicidal ideation severity in a sample of acute-care psychiatric inpatients. Participants were comprised of 119 adults (42.9% women; Mage = 33.3, SD = 11.0) admitted to a public, university-affiliated, psychiatric acute-care inpatient hospital in a large metropolitan area. Results indicated significant interactive effects of PTSD symptom severity and observing (ΔR 2 = 0.032, β = 0.19, p < 0.05), acting with awareness (ΔR 2 = 0.071, β = − 0.27, p = 0.002), and nonjudging of inner experience (ΔR 2 = 0.066, β = − 0.27, p = 0.002) with regard to suicidal ideation severity. These effects were documented after controlling for theoretically relevant covariates, including number of traumatic event types experienced and number of prior suicide attempts. A significant main effect was also present for PTSD symptom severity with regard to suicidal ideation severity. In addition, there was a significant main effect for nonjudging of inner experience with regard to suicidal ideation severity; no other mindfulness facets demonstrated significant main effects with regard to suicidal ideation severity.