Background: Heterosexism has been identified as being a contributing factor of suicidality in sexual minority adults (SMA), and social support is believed to be important for mitigating these effects. Research evaluating racial/ethnic differences in suicidality among SMA is limited despite racial/ethnic minorities being at greater risk. Aims: We aimed to examine the associations between heterosexism, suicidal ideation, and social support in a sample of racially/ethnically diverse SMA. Method: SMA (N = 239) were recruited as part of an online survey on sexuality and health based in the United States. Results: There were significant positive main effects of heterosexism and significant negative main effects for non-White racial/ethnic identity on suicidal ideation. There were significant negative main effects for social support from family and a significant other but not from friends. A significant interaction of social support from a significant other and racial ethnic identity was qualified by a significant three-way interaction with heterosexism. Social support from a significant other buffered the effect of heterosexism on suicidal ideation among non-White but not among White SMA. All other interactions were not significant. Limitations: Within-group differences of racial/ethnic groups and other domains of social support were not examined, and the cross-sectional nature of the data precludes causal inference. Conclusion: Support from a significant other may be important for suicidality, particularly for racial/ethnic minority SMA.