Suicidal ideation (SI) is a known precursor to suicide among men. While loneliness and thwarted belongingness (TB) have been identified as key factors influencing SI, no study has reported on all three constructs to investigate whether loneliness is associated with SI by way of TB. Furthermore, it is not clear whether personality impairment has a moderating role on this process. The present study examined the impact of loneliness on SI among men and whether TB mediated this relationship. Additionally, the study investigated whether personality impairment (i.e., self-functioning, interpersonal functioning) moderated the relationship between loneliness and TB. Canadian men (N = 434) completed an online survey that included self-report assessments of the study constructs. Conditional process modeling was used to test the indirect effect of loneliness on SI via the mediating effect of TB. Findings indicated a significant association between loneliness and SI that was mediated by TB. Further, impairment in self-functioning moderated the relationship between loneliness and TB, indicating that the relationship was stronger among men with greater difficulties in self-functioning. The findings are important to consider within the COVID-19 context, as they point to the need to reduce the detrimental impacts of loneliness, thereby potentially mitigating male SI.