Transgender people experience an excess burden of child sexual abuse (CSA), mental health concerns, and substance use compared to cisgender populations. Posttraumatic stress disorder (PTSD) has been found to mediate the association between CSA and substance use behaviors in cisgender populations, but this dynamic has not been previously examined among transgender adults. The aim of this study is to test if PTSD may mediate a relationship between CSA and substance use among transgender adults. Data were analyzed from the U.S. Transgender Population Health Survey (2016-2018), a national probability sample of transgender adults (N = 274). CSA was measured using the Adverse Childhood Experiences subsection for sexual abuse. Past-month PTSD was measured using the Primary Care-PTSD Diagnostic and Statistical Manual of Mental Disorders, fourth edition screening tool. Substance use was determined by lifetime binge drinking, polydrug use, and the Drug Use Disorders Identification Test. Baron and Kenny's approach was used to assess PTSD as a mediator between CSA and substance use. Within our sample, nearly half (45%) of the transgender adults experienced CSA. Lifetime binge drinking (40%), polydrug use (20%), and indications of drug-use-related problems (Drug Use Disorders Identification Test x¯ = 4.52) were frequently reported. Transgender adults who have experienced CSA had increased risk of PTSD and substance use, and PTSD was a mediator in all models. Results suggest that adult transgender CSA survivors are at increased risk of drug and alcohol use, and that PTSD may be an important contextual factor for substance use. (PsycInfo Database Record (c) 2024 APA, all rights reserved).