Background and purpose Formerly incarcerated people die by suicide at a rate 6.75 times higher than the general population. In this study we ask whether existential isolation, the feeling of being alone in one’s subjective experiences, offers new insights into this staggering statistic. We specifically ask whether existential isolation predicts suicidal ideation among formerly incarcerated individuals even after accounting for their levels of interpersonal isolation, emotional support, and instrumental support. Method We examined cross-sectional survey data from American participants (n = 946) recruited over the Amazon Mechanical Turk (MTurk) platform. Results Consistent with prediction, we found that people with an incarceration history reported more existential isolation, but not more interpersonal isolation, than those without an incarceration history. A hierarchical regression analysis revealed that existential isolation explained additional variance in suicidal ideation beyond what was explained by interpersonal isolation, instrumental support, and emotional support. Fisher’s r-to-z transformations demonstrated that the magnitude of the relationship between existential isolation and suicidal ideation was stronger among those with an incarceration history than among those without. Finally, existential isolation mediated the relationship between incarceration and suicidal thoughts, even when holding constant other predictors of suicidal ideation. Conclusion Reentry programs may target existential isolation among formerly incarcerated individuals to improve emotional wellbeing.