Background: Childhood trauma and affective disorders are known risk factors for adult suicidal behavior. Studies have shown a mediating effect of insecure attachment on the effect of childhood trauma and suicidal behavior but so far it is not clear whether this effect is related to an attachment dimension (anxiety, avoidance).Aim: The present study sought to examine the mediating effect of attachment anxiety and avoidance on suicidal behavior.Methods: We analyzed data on childhood trauma, attachment style, depression severity, presence of prior suicide attempts and current suicide ideation from 96 patients diagnosed with an affective disorder. Two mediation analyses were conducted to assess the effect of childhood trauma on 1) prior suicide attempts and 2) current suicidal ideation through its effect on attachment.Results: We found that childhood trauma had a complete mediated effect on the presence of prior suicide attempts through its effect on avoidant attachment (a1b1 = 0.0120, 95%-CI [0.0031, 0.0276]). However, only emotional abuse had a direct influence on suicidal ideation (c' = 0.0273, p < 0.01) without any indirect effect of anxious or avoidant attachment.Limitations: Variables were not assessed in a prospective way and sample size was small.Conclusions: Our findings suggest that individuals with avoidant attachment and childhood trauma are likely to present a high suicide risk. Since avoidant attachment is associated with altered perceptions and eventual rejection of social support, we recommend to screen for attachment early and to engage patients in therapeutical approaches focusing on the client-therapist alliance.