Insomnia can be lethal, increasing the risk of suicide and accidental death by overdose. In this review we present a synthesis of the literature investigating the insomnia-suicide link and the psychological mechanisms underpinning the association. Specifically, we review the findings of prospective epidemiological studies demonstrating the insomnia-suicide link in adult and adolescent community populations. Robust associations between insomnia and suicide are observed in clinically depressed and anxious populations, and there are indications across a number of cross-sectional and longitudinal studies that these linkages are attributable to a disrupted sleep pattern, dysfunctional beliefs about sleep and nightmares, independent of depression and anxiety symptoms. Borderline personality disorder (BPD) and chronic pain (CP) are highlighted as high-risk subgroups given the elevated rates of insomnia and suicidality in both conditions. Aside from the influence of comorbid depression and anxiety symptoms, emerging evidence has identified impulsivity and emotional dysregulation as possible mechanisms driving the insomnia-suicide link in BPD, and catastrophizing and the sense of defeat/entrapment as potential cognitive pathways through which insomnia aggravates suicidality in CP. Screening for, and interventions that tackle, insomnia and these associated psychological mechanisms, offer a novel avenue for reducing suicidality across a range of clinical and non-clinical populations.