Background: Alcohol use and drug injection are prevalent among homeless youths. The aim of this study was to identify the associated factors of alcohol consumption and drug injection among homeless youths aged 18-29 years. Methods: Data on 202 homeless youths (111 males and 91 females) were collected using a standardized questionnaire and face-to-face interview. Lasso logistic regression was applied to determine the impact of associated factors on alcohol consumption and drug injection. Findings: The mean age of the participants was 26.30 ± 3.19 years. Also, the prevalence of alcohol consumption and drug injection was 33.0% [95% confidence interval (CI): 30-36] and 4.0% (95% CI: 0-8), respectively; 6 people (3.0%) consumed alcohol and injected drugs at the same time. Correlates of alcohol consumption and drug injection were male sex [odds ratio (OR)Alc = 5.7], age (ORAlc = 0.96 and ORDI = 0.98), bachelor or higher education level (ORAlc = 1.34), non-Iranian nationality (ORAlc = 0.05 and ORDI = 0.18), food score (ORDI = 0.92), smoking (ORAlc = 2.05), substance use (ORAlc = 1.12), opposite sex relationship (ORAlc = 1.6), homosexual relationship (ORAlc = 3.56 and ORDI = 2.69), and mental disorder (ORAlc = 0.99). Conclusion: Based on our findings, it seems that the homeless youth are more desired to use alcohol and drug injection, whereas the prevalence of alcohol consumption and drug injection in homeless youth was higher than general youth population in Iran. Therefore, some suitable solutions are needed to prevent the homelessness. Also, the effective variables that were identified in this study for alcohol use and drug injection can help design and implement beneficial interventions.