•Social isolation is strongly associated with suicidal outcomes.•The subjective feeling of loneliness has a major impact, even transculturally.•Objective and subjective social isolation should be added in suicide risk assessment. Social isolation is one of the main risk factors associated with suicidal outcomes. The aim of this narrative review was to provide an overview on the link between social isolation and suicidal thoughts and behaviors. We used the PubMed database to identify relevant articles published until April 13, 2018. We focused on: (a) systematic reviews, meta-analyses, and narrative reviews; (b) original observational studies with large samples (N ≥ 500); and (c) qualitative studies. We included all relevant suicidal outcomes: suicidal ideation (SI), suicidal planning, non-suicidal self-injury, deliberate self-harm, suicide attempt (SA), and suicide. The main social constructs associated with suicidal outcomes were marital status (being single, separated, divorced, or widowed) and living alone, social isolation, loneliness, alienation, and belongingness. We included 40 original observational studies, the majority of them performed on adolescents and/or young adults (k = 23, 57.5%). Both the objective condition (e.g., living alone) and the subjective feeling of being alone (i.e., loneliness) were strongly associated with suicidal outcomes, in particular with SA and SI. However, loneliness, which was investigated in most studies (k = 24, 60%), had a major impact on both SI and SA. These associations were transculturally consistent. Confounding factors can limit the weight of the results obtained in observational studies. Data from the observational studies suggest that both objective social isolation and the subjective feeling of loneliness should be incorporated in the risk assessment of suicide. Interventional studies targeting social isolation for suicide prevention are needed.