Objective: Despite preliminary evidence of unique acute cognitive and psychopharmacological changes attributable to combined alcohol and cannabis use, few studies have investigated more chronic effects of same-day co-use, particularly during neurodevelopmentally sensitive periods. Therefore, relationships between past-month binge alcohol and cannabis co-use and cognitive functioning were examined in adolescents and young adults. Method: Data from the Imaging Data in Emerging Adults with Addiction (IDEAA) Consortium were used to assess cognitive functioning in emerging adults with a large range of substance use (n =232; 15–26 years old) who were abstinent for at least 3weeks. Multiple regressions assessed cognitive functioning by past-month binge episodes, cannabis use episodes, and same-day co-use, controlling for covariates (e.g., study site, sex, age). Results: After correcting for multiple comparisons, more past-month co-use episodes were related to decreased Ruff 2&7 selective attention accuracy (p = .036). Sex significantly covaried with California Verbal Learning Test– Second Edition initial learning. Conclusions: Although few significant relationships were found and effect sizes are modest, the persistence of an effect on attention despite a period of sustained abstinence highlights the need to carefully investigate patterns of substance use and potential independent and interactive effects on the developing brain.