Alcohol use disorder (AUD) and posttraumatic stress disorder (PTSD) frequently co-occur, highlighting the importance of understanding the etiology of these comorbid conditions. Although AUD and PTSD are moderately heritable with modest overlap in genetic risk as estimated from family studies, there has been a paucity of work using molecular genetic data to estimate shared genetic effects on these conditions. This study used large-scale genomewide molecular data to examine shared genetic risk for AUD, specifically alcohol dependence (AD), and PTSD through cross-trait linkage disequilibrium (LD) score regression (LDSC; also known as LDSR). Summary statistics came from the Psychiatric Genomics Consortium (PGC) PTSD Workgroup Freeze 2 European ancestry (EA) participants ( = 174,659) and AD summary statistics in EA participants ( = 38,686) came from the PGC Substance Use Disorders (SUD) Workgroup. We performed LDSC to estimate genetic correlation between AD and PTSD using HapMap3 variants and LD scores from the 1000 Genomes project. A moderate, significant correlation was observed between AD and PTSD ( = .35, = .02), with sex differences identified through stratified analyses. Our results are the first to demonstrate evidence of a shared molecular genetic etiology for AD and PTSD. Further research is needed to better understand possible sex differences in shared heritability and extend these results to additional populations. (PsycINFO Database Record (c) 2020 APA, all rights reserved).