Violence and harassment toward transgender women are associated with suicidal thoughts and behaviors, and social support might moderate such association. This analysis explored the association between certain forms of violence and harassment and suicidal ideation and moderation by social support. Better understanding of these associations could guide mental health services and structural interventions appropriate to lived experiences of transgender women. This cross-sectional analysis used data from CDC's National HIV Behavioral Surveillance Among Transgender Women. During 2019-2020, transgender women were recruited via respondent-driven sampling from seven urban areas in the United States for an HIV biobehavioral survey. The association between experiencing certain forms of violence and harassment (i.e., gender-based verbal and physical abuse or harassment, physical intimate partner abuse or harassment, and sexual violence) and suicidal ideation was measured using adjusted prevalence ratios and 95% CIs generated from log-linked Poisson regression models controlling for respondent-driven sampling design and confounders. To examine moderation, the extents of social support from family, friends, and significant others were assessed for interaction with certain forms of violence and harassment; if p interaction was <0.05, stratified adjusted prevalence ratios were presented. Among 1,608 transgender women, 59.7% experienced certain forms of violence and harassment and 17.7% reported suicidal ideation during the past 12 months; 75.2% reported high social support from significant others, 69.4% from friends, and 46.8% from family. Experiencing certain forms of violence and harassment and having low-moderate social support from any source was associated with higher prevalence of suicidal ideation. Social support from family moderated the association between experiencing certain forms of violence and harassment and suicidal ideation (p interaction = 0.01); however, even in the presence of high family social support, experiencing certain forms of violence and harassment was associated with higher prevalence of suicidal ideation. Social support did not completely moderate the positive association between experiencing violence and harassment and suicidal ideation. Further understanding of the social support dynamics of transgender women might improve the quality and use of social support. Policymakers and health care workers should work closely with transgender women communities to reduce the prevalence of violence, harassment, and suicide by implementing integrated, holistic, and transinclusive approaches.