Little is known about the social networks of homeless youth in emerging adulthood despite the importance of this information for interventions to reduce health risks. This study examined the composition of social networks, and the risks and supports present within them, in a random sample of 349 homeless youth (33.4% female, 23.9% African American, 17.7% Hispanic) between the ages of 18 and 24. Social network members who were met on the street were among the most likely to be perceived as engaging in risky sex, as well as to engage in substance use with the youth. Youth were more likely to count on relatives and sex partners for support compared to other network members, but they also were more likely to use substances with sex partners and perceived them as engaging in risky sex. Interventions may need to recognize the importance of intimate relationships during the developmental stage of emerging adulthood by enhancing supportive bonds and reducing substance use and risky sex in these relationships.