Human laboratory studies and twin research investigating relationships between alcohol use/pathology and gambling generally have yielded contradictory results, sometimes suggesting causal relationships and common genetic risk factors. 2860 individuals (mean age: 25.60, s.d = 3.21, 50.62% female) from separate clinical (n = 636) and community based (twin) samples (n = 2224) were used to assess associations between past year alcohol use and frequency of past year gambling behaviors (gambling frequency). After adjustment for demographic and psychiatric covariates, individual-level analyses detected that increased alcohol use was associated with more frequent gambling behaviors in twin and clinical samples. Co-twin control models were then used to test potential causal (direct) relationships between alcohol use and gambling frequency. Controlling for all covariates and shared genetic/environmental factors, we found increased alcohol use directly predicted more frequent gambling behaviors (consistent with causality). Our study also suggests shared genetic and/or environmental risk factors contribute to the association between increased alcohol use and frequent gambling behavior, a finding that may be more pronounced in males. The present study helps bridge the gap between twin research and human laboratory studies on gambling and alcohol use and corroborates findings across community and clinical samples. Overall, our findings support both common risk factors between alcohol use and gambling as well as a direct relationship between alcohol use and gambling frequency. Recognizing these dual processes could prove useful for gambling-related prevention/intervention programs.